nir: Document a couple instances of parent_instr

nir: Document a couple instances of parent_instr

nir_ssa_def::parent_instr and nir_src::parent_instr have the same name,
but they mean really different things.  I choose to save the next person
the hour+ that I just spent figuring that out.  Even now that I know, I
doubt I'd notice in code review that someone typed foo->parent_instr
when they actually meant foo->ssa->parent_instr.

v2: Minor wording tweak in nir_ssa_def::parent_instr.  Suggested by
Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>