Document RigidBodies in character mode never sleeping automatically

[Calinou]

See #7996.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.3.

[chucklepie]

Hello, I don't think the documentation for this is fully correct:

[b]Note:[/b] A RigidBody2D will never enter sleep mode automatically if its [member mode] is [constant MODE_CHARACTER]. It can still be put to sleep manually by setting its [member sleeping] property to [code]true[/code].

If sleeping is set to false then setting MODE_CHARACTER turns sleeping to true. I think there should be a documentation update for sleeping not (just) can_sleep.