Document RigidBodies in character mode never sleeping automatically

See #7996.

(cherry picked from commit 5fbf709ca22e0d8b7db885d96cd059e7c39528fa)

doc/classes/RigidBody.xml

@@ -160,6 +160,7 @@
 		</member>
 		<member name="can_sleep" type="bool" setter="set_can_sleep" getter="is_able_to_sleep" default="true">
 			If [code]true[/code], the body can enter sleep mode when there is no movement. See [member sleeping].
+			[b]Note:[/b] A RigidBody3D 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].
 		</member>
 		<member name="contact_monitor" type="bool" setter="set_contact_monitor" getter="is_contact_monitor_enabled" default="false">
 			If [code]true[/code], the RigidBody will emit signals when it collides with another RigidBody. See also [member contacts_reported].

doc/classes/RigidBody2D.xml

@@ -133,6 +133,7 @@
 		</member>
 		<member name="can_sleep" type="bool" setter="set_can_sleep" getter="is_able_to_sleep" default="true">
 			If [code]true[/code], the body can enter sleep mode when there is no movement. See [member sleeping].
+			[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].
 		</member>
 		<member name="contact_monitor" type="bool" setter="set_contact_monitor" getter="is_contact_monitor_enabled" default="false">
 			If [code]true[/code], the body will emit signals when it collides with another RigidBody2D. See also [member contacts_reported].