[WIP] Improve SoftBody documentation

**Your Godot version:** 3.2.2

**Issue description:**

There are a number of undocumented properties in the `SoftBody`/`SoftBody3D` class--the underlying implementation of which is the Bullet physics library:

- [ ] `areaAngular_stiffness`

- [ ] `damping_coefficient`

- [ ] `drag_coefficient`

- [ ] `linear_stiffness`

- [ ] `pose_matching_coefficient`

- [ ] `pressure_coefficient`

- [ ] `volume_stiffness`

While being distracted by trying to figure out more about SoftBody I encountered some potential useful sources of information for documenting these properties (& others):

* [Soft Body properties.pdf (attached)](https://github.com/godotengine/godot-docs/files/4961043/Soft.Body.properties.pdf) (by `dphil` [via](https://pybullet.org/Bullet/phpBB3/viewtopic.php?p=24280#p24280))

* [TODO]

**URL to the documentation page (if already existing):**

* https://github.com/godotengine/godot/blob/938e2e966fb92ce51656b8d50b37bebf256fb6fb/doc/classes/SoftBody3D.xml (4.0)

* https://github.com/godotengine/godot/blob/3.2/doc/classes/SoftBody.xml (3.2 [a.k.a.](https://github.com/godotengine/godot/blob/bdd1e74869cab44c062994cb8a5b3068fa7164d2/doc/classes/SoftBody.xml))

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