py/obj.h: Clarify comments about mp_map_t is_fixed and is_ordered.
py/obj.h: Clarify comments about mp_map_t is_fixed and is_ordered.

Long ago, prior to 0ef01d0a75b8b2f48a72f0041e048a390b9e75b6, fixed and
ordered maps were the same setting with the "table_is_fixed_array" member
of mp_map_t.  But these settings are actually independent, and it is
possible to have is_fixed=1, is_ordered=0 (although this can currently
only be done by tools/cc1).  So update the comments to reflect this.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython