docs/phonemes.md: add the vwl feature to the manner of articulation table.

docs/phonemes.md

 | click                  | `clk`     |        | `vlc igs orl`         |
 | ejective               | `ejc`     |        | `vlc igs orl occ`     |
 | implosive              | `imp`     | ◌ʼ     | `gtc igs`             |
+| vowel                  | `vwl`     |        | `pmc egs orl vow`     |
 
 The features for these manners of articulation are provided for convenience,
 and to make it easier to describe the IPA consonants. Internally, the
 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`.
 
+The `vwl` phonemes are described using vowel height and backness features,
+while consonants (the other manners of articulation) are described using
+place of articulation features.
+
 Additionally, the manner of articulation can be refined using the following
 features:
 
 | `fla`   | flap              | A single tap of the tongue against the secondary articulator.                     |
 | `tri`   | trill             | A rapid vibration of the primary articulator against the secondary articulator.   |
 | `app`   | approximant       | The vocal tract is narrowed at the place of articulation without being turbulant. |
+| `vow`   | vowel             | The phoneme is articulated as a vowel instead of a consonant.                     |
 
 ### Phonation
 
 
 ### Vowels
 
-| Feature | Kirshenbaum | Name            |
-|---------|-------------|-----------------|
-| `vwl`   | `vwl`       | vowel           |
-
 #### Height
 
 | Feature | Kirshenbaum | Name                   |