Fix physics server force integration docs

[rburing]

Removed incorrect information, added correct information. Based on #79977 (comment) and added more details.

cc @mihe

[Mickeon]

Similar suggestions for the PhysicsServer3D equivalents, too

[rburing]

Thanks for the thorough reviews! I've standardized on "standard force integration" now, as a natural counterpart to "custom" (the word which I also added in the documentation now, inspired by the documentation of rendering callbacks).

Note PhysicsDirectBodyState2D.integrate_forces does something different, which is now also documented correctly.

Edit: I'm not a huge fan of the (pre-existing) "gravity and air friction" mention as it is, please feel free to suggest an improvement.

[mihe]

The "air friction" part should probably be replaced with "damping", like you've already done with PhysicsDirectBodyState*D.integrate_forces.

[akien-mga]

Thanks!