docs/phonemes.md: improve the specification of the manners of articulation.

docs/phonemes.md

   - [Vowels](#vowels)
   - [Positioning Diacritics](#positioning-diacritics)
 - [Features](#features)
-  - [Air Flow](#air-flow)
-  - [Initiator](#initiator)
+  - [Manner of Articulation](#manner-of-articulation)
+    - [Air Flow](#air-flow)
+    - [Initiator](#initiator)
+    - [Target](#target)
+    - [Manner](#manner)
   - [Phonation](#phonation)
   - [Place of Articulation](#place-of-articulation)
     - [Active Articulators](#active-articulators)
     - [Passive Articulators](#passive-articulators)
   - [Rounding and Labialization](#rounding-and-labialization)
-  - [Consonants](#consonants)
-    - [Manner of Articulation](#manner-of-articulation)
   - [Vowels](#vowels-1)
     - [Height](#height)
     - [Backness](#backness)
 
 ## Features
 
-### Air Flow
+### Manner of Articulation
 
-| Feature | Symbol | Name       | Description                                            |
-|---------|--------|------------|--------------------------------------------------------|
-| `egs`   | ↑      | egressive  | The air flow is moving outwards from the initiator.    |
-| `igs`   | ↓      | ingressive | The air flow is moving inwards towards the initiator.  |
+The manner of articulation is described in terms of several distinct feature
+types. The possible manners of articulation are:
+
+| Manner of Articulation | Feature   | Symbol | Features              |
+|------------------------|-----------|--------|-----------------------|
+| nasal                  | `nas`     |        | `pmc egs nsl occ`     |
+| plosive (stop)         | `stp`     |        | `pmc egs orl occ`     |
+| affricate              | `afr`     |        | `pmc egs orl occ frr` |
+| fricative              | `frc`     |        | `pmc egs orl frv`     |
+| tap/flap               | `flp`     |        | `pmc egs orl fla`     |
+| trill                  | `trl`     |        | `pmc egs orl tri`     |
+| approximant            | `apr`     |        | `pmc egs orl app`     |
+| click                  | `clk`     |        | `vlc igs orl`         |
+| ejective               | `ejc`     |        | `vlc igs orl occ`     |
+| implosive              | `imp`     | ◌ʼ     | `gtc igs`             |
+
+The features for these manners of articulation are provided for convenience,
+and to make it easier to describe the IPA consonants. Internally, the
+distinct feature types are used.
+
+For `imp` consonants, they use the features of the base phoneme except for
+the `pmc` and `egs` features. Thus, a `nas imp` is a `gtc igs nsl occ`.
+
+Additionally, the manner of articulation can be refined using the following
+features:
+
+| Feature | Name     | Description                                                 |
+|---------|----------|-------------------------------------------------------------|
+| `lat`   | lateral  | The air flow is directed along the sides of the tongue.     |
+| `sib`   | sibilant | The air flow is directed through the teeth with the tongue. |
+
+#### Air Flow
+
+| Feature | Symbol | Name       | Description                                                       |
+|---------|--------|------------|-------------------------------------------------------------------|
+| `egs`   | ↑      | egressive  | The air flow is moving outwards from the initiator to the target. |
+| `igs`   | ↓      | ingressive | The air flow is moving inwards from the target to the initiator.  |
 
 The ↑ and ↓ symbols are from the extended IPA<sup>\[<a href="#ref7">7</a>\]</sup>.
 They only need to be used when the air flow is different to the base IPA
 phoneme (e.g. using ↓ on pulmonic consonants).
 
-### Initiator
+#### Initiator
 
 | Feature | Name       | Description                                                           |
 |---------|------------|-----------------------------------------------------------------------|
 | `vlc`   | velaric    | The velum is closed and the tongue is used to generate the airstream. |
 | `pcv`   | percussive | There is no airstream used to produce this sound.                     |
 
-#### Phonation
+#### Target
+
+| Feature | Name       | Description                                     |
+|---------|------------|-------------------------------------------------|
+| `nsl`   | nasal      | The air flows through the nose.                 |
+| `orl`   | oral       | The air flows through the mouth.                |
+
+#### Manner
+
+| Feature | Symbol | Name       | Description                                      |
+|---------|--------|------------|--------------------------------------------------|
+| `occ`   |        | occlusive  | The air flow is blocked within the vocal tract.  |
+| `frv`   |        | fricative  | The air flow is constricted, causing turbulence. |
+
+### Phonation
 
 The phonation features describe the degree to which the glottis (vocal chords) are open or closed.
 
 annotations to specify the type of labialization. Vowels can use these to
 change their labialization from the default one specified by `rnd`.
 
-### Consonants
-
-#### Manner of Articulation
-
-| Feature | Kirshenbaum | Name            |
-|---------|-------------|-----------------|
-| `stp`   | `stp`       | plosive (stop)  |
-| `nas`   | `nas`       | nasal           |
-| `trl`   | `trl`       | trill           |
-| `flp`   | `flp`       | tap/flap        |
-| `afr`   | `afr`       | affricate       |
-| `frc`   | `frc`       | fricative       |
-| `apr`   | `apr`       | approximant     |
-| `clk`   | `clk`       | click           |
-| `ejc`   | `ejc`       | ejective        |
-| `imp`   | `imp`       | implosive       |
-
-The following features can be combined with other manners of articulation:
-
-| Feature | Kirshenbaum | Name            |
-|---------|-------------|-----------------|
-| `lat`   | `lat`       | lateral         |
-| `sib`   |             | sibilant        |
-
-__NOTE:__ `sib` is not used in the IPA charts.
-
 ### Vowels
 
 | Feature | Kirshenbaum | Name            |