[WIP] Improve SoftBody documentation

[follower]

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) (by dphil via)

• [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.)

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[jayypluss]

Also pinned_points: PoolIntArray is not documented yet.

Godot version: 3.2.1

[Calinou]

Also pinned_points: PoolIntArray is not documented yet.

Godot version: 3.2.1

The property is registered dynamically, which means --doctool has no chance to list it in the class reference: https://github.com/godotengine/godot/blob/83d0e97e03b097310963dff76a9bde24bd49ed13/scene/3d/soft_body.cpp#L177