DOC: docstring fixes for set_{q/s}form methods

set_qform wrongly refered to sform code.  Make explicit that old header
code used in None case.

Remove listing of self as parameter in {q/s}form methods and some
others.
BF+TST: set_{s/q}form deals with None affine

set_sform / set_qform causing error when setting affine that is None.