docs/phonemes.md: improve the rounding and labialization specification and use these to describe the co-articulated consonants.

docs/phonemes.md

   - [Place of Articulation](#place-of-articulation)
     - [Active Articulators](#active-articulators)
     - [Passive Articulators](#passive-articulators)
+  - [Rounding and Labialization](#rounding-and-labialization)
   - [Consonants](#consonants)
-    - [Co-articulation](#co-articulation)
     - [Manner of Articulation](#manner-of-articulation)
   - [Vowels](#vowels-1)
     - [Height](#height)
     - [Backness](#backness)
-    - [Rounding](#rounding)
   - [Diacritics](#diacritics)
     - [Syllabicity](#syllabicity)
     - [Consonant Release](#consonant-release)
 
 ### Other Symbols
 
-| Symbol | Features                |
-|--------|-------------------------|
-| ʍ      | `vls` `lbv` `apr`       |
-| w      | `mdv` `lbv` `apr`       |
-| ɥ      | `mdv` `lbp` `apr`       |
-| ɧ      | `vls` `vzd` `pla` `frc` |
+| Symbol | Alternative | Features                |
+|--------|-------------|-------------------------|
+| ʍ      | ɰ̊ʷ           | `vls` `vel` `ptr` `apr` |
+| w      | ɰʷ          | `mdv` `vel` `ptr` `apr` |
+| ɥ      | jʷ          | `mdv` `pal` `ptr` `apr` |
+| ɧ      |             | `vls` `vzd` `pla` `frc` |
 
 ### Vowels
 
 | `egs`   | epiglottis             |
 | `gts`   | glottis                |
 
-### Consonants
+### Rounding and Labialization
 
-#### Co-articulation
+| Feature | Symbol | Name       | Rounded | Position                                |
+|---------|--------|------------|---------|-----------------------------------------|
+| `unr`   |        | unrounded  | No      | Close to the jaw.                       |
+| `ptr`   | ◌ʷ, ◌ᶣ | protruded  | Yes     | Protrude outward from the jaw.          |
+| `cmp`   | ◌ᵝ     | compressed | Yes     | Close to the jaw.                       |
+| `rnd`   |        | rounded    | Yes     | `ptr` if `bck` or `cnt`; `cmp` if `fnt` |
 
-| Feature | Kirshenbaum | Name            |
-|---------|-------------|-----------------|
-| `lbv`   | `lbv`       | labial-velar    |
-| `lbp`   |             | labial-palatal  |
+The `unr` and `rnd` features are used for vowels to describe their default
+labialization. Consonants are `unr` by default, and can use the ◌ʷ, ◌ᶣ and ◌ᵝ
+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
 
 | `cnt`   | `cnt`       | center          |
 | `bck`   | `bck`       | back            |
 
-#### Rounding
-
-| Feature | Kirshenbaum | Name            |
-|---------|-------------|-----------------|
-| `unr`   | `unr`       | unrounded       |
-| `rnd`   | `rnd`       | rounded         |
-
 ### Diacritics
 
 #### Syllabicity
 |---------|-------------|-----|-----------------|
 | `mrd`   |             | ◌̹   | more rounded    |
 | `lrd`   |             | ◌̜   | less rounded    |
-| `lzd`   | `lzd`       | ◌ʷ  | labialized or labio-velarized |
 | `pzd`   | `pzd`       | ◌ʲ  | palatalized     |
 | `vzd`   | `vzd`       | ◌ˠ  | velarized       |
 | `fzd`   | `fzd`       | ◌ˤ  | pharyngealized  |