docs/phonemes.md: restructure the remaining diacritic features.

docs/phoneme_model.md

   - [Vowel Rounding](#vowel-rounding)
 - [Syllabicity](#syllabicity)
 - [Consonant Release](#consonant-release)
+- [Tongue Root](#tongue-root)
+- [Fortis and Lenis](#fortis-and-lenis)
 - [References](#references)
 
 ----------
 | `lrs`   | lateral release                 |
 | `unx`   | no audible release (unexploded) |
 
+## Tongue Root
+
+The tongue root position can be specified using the following features:
+
+| Feature | Symbol | Name                  |
+|---------|--------|-----------------------|
+| `atr`   | ◌̘      | advanced tongue root  |
+| `rtr`   | ◌̙      | retracted tongue root |
+
+## Fortis and Lenis
+
+| Feature | Name   |
+|---------|--------|
+| `fts`   | fortis |
+| `lns`   | lenis  |
+
 ## References
 
 1. <a name="ref1"></a> Kirshenbaum, Evan,

docs/phonemes.md

   - [Syllabicity](#syllabicity)
   - [Consonant Release](#consonant-release)
   - [Co-articulation](#co-articulation)
-- [Features](#features)
+  - [Tongue Root](#tongue-root)
   - [Fortis and Lenis](#fortis-and-lenis)
   - [Suprasegmentals](#suprasegmentals)
     - [Stress](#stress)
 | `nzd`   | ◌̃      | nasalized      |
 | `rzd`   | ◌˞     | rhoticized     |
 
-## Features
+### Tongue Root
 
-Additionally, the tongue root position can be specified using the following
-features:
+The tongue root position can be specified using the following features:
 
 | Feature | Symbol | Name                  |
 |---------|--------|-----------------------|