Add missing descriptions to `PhysicalBone3D` class.

[Manik2607]

Documentation Enhancements:

• doc/classes/PhysicalBone3D.xml: Added detailed descriptions for methods such as apply_impulse, get_bone_id, and is_simulating_physics to explain their functionality and usage.
• doc/classes/PhysicalBone3D.xml: Provided descriptions for constants like JOINT_TYPE_NONE, JOINT_TYPE_PIN, and JOINT_TYPE_6DOF to specify the types of joints that can be applied to PhysicalBone3D.dot documentation.

I have tried to be constant with other methods in godot. Example : apply_impulse documentation is similar to apply_impulse in PhysicsBody3D. Its my second contribution let me know if any changes are needed.

[Manik2607]

Do I need to open an issue that documentation is missing for this to work or not? And it possible to work on other issues while this one is for review?

[Mickeon]

I already answered on RocketChat, but just for transparency

Do I need to open an issue that documentation is missing for this to work or not?

No reason to. Yes if the documentation was debatably inaccurate, but there was no documentation to speak of.

And it possible to work on other issues while this one is for review?

Of course

[Repiteo]

Thanks!