New semantic analyser: Establish and document the defer() calling convention

[ilevkivskyi]

(adding documentation label because it is also important for external plugin contributors)

The most important point is probably to decide on which side of anal_type() (and all other similar functions) we should call defer(). I think there are still some calls that are not needed (and may be even harmful during final iteration, note that the defer() calls in typeanal.py are correctly guarded for final iteration).

See also #6725