For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
New property disable_mode to set different behaviors:
Remove: remove from physics simulation
MakeStatic: change body mode to static (doesn't affect area and soft body)
KeepActive: do nothing
Extra change:
Handle disable/enable node state with specific notifications, in order
to differentiate global pause from disabled nodes.
