Update area_create description in PhysicsServer2D/3D to clear up possible confusions

[TheKiromen]

I'm a newbie who never worked with game engine before so apologies in advance if this is incorrect.

I spent way too long trying to figure out why collisions with areas created from Area2D nodes were working but not between areas created using PhysicsServer2D.area_create().

From my testing it looks like by default the monitorable property is set to false, which can be confusing since there is no easy way to check it (no area_get_monitorable() method for example). Especially since areas created with Area2D nodes have this enabled by default.

To reduce chances other people will be confused like me I updated the description of the method to explicitly state that.

I didn't mention monitoring property since areas are always monitoring, and disabling it in Area2D node just means setting empty callback. If you think there may be value in adding this as well, feel free to suggest changes.

[rburing]

Looks good to me. Please copy the whole description to the method with the same name in PhysicsServer3D (and replace 2D by 3D), and I can approve the PR.

If you have time, please add the same information to body_create in both the 2D and 3D physics server.

Changes needed

[TheKiromen]

Updated description for area_create() and body_create() int both PhysicsServer2D and 3D.

Since body doesn't have monitoring/monitorable properties I mentioned the default BodyMode there

Needs new work

[rburing]

Looks good to me.

[AThousandShips]

Please squash your commits into one, see here

[AThousandShips]

Please clean up the commit message so it's just the necessary information, like the PR title, excluding all the "Update X" messages

[TheKiromen]

I think I managed to do it, should I create a separate PR for body_create changes?

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉