Add documentation to PhysicsServer2DExtension

[Mickeon]

Oh goodness me.

This PR... This PR fills in the class reference of PhysicsServer2DExtension.

A grand majority of these methods link to the PhysicsServer2D equivalents. The reasons for this are as follow:

• Curating whole new descriptions with the assumption that that they are overridable now is not something I would wish for to my dearest enemy. It could be my job and I wouldn't do it, at least not all at once.
• Copying the descriptions between the two word-by-word would mean having to maintain two copies of the same identical description.
• This documentation is tailored towards advanced users, even more "advanced" than those that would read PhysicsServer2D's class reference. They should roughly know what they're doing already.
  • Most of this documentation basically behaves like a quality of life "click here to read more".

Not all of them point back to PhysicsServer2D. Some have been described from scratch, because there's no equivalent of them exposed to the scripting languages.

Could use the feedback of physics-savvy maintainers, but I somehow have the confidence that the new descriptions are accurate enough.

[Mickeon]

This and #87030 could still use a look

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.

[akien-mga]

Cherry-picked for 4.1.4.