Fix REALLOCATE_BITMAPSETS code

7d58f2342 added a compile-time option to have bitmapset.c reallocate the
set before returning when a set is modified.  That commit failed to do
its job in various cases and returned the input set when it shouldn't
have in these cases.  Here we fix those missing cases.

This commit also adds some documentation about what
REALLOCATE_BITMAPSETS is for.  This is important as future functions
that go inside bitmapset.c need to know if they need to do anything
special when this compile-time option is defined.

Also, between 71a3e8c43 and 7d58f2342 some Asserts seem to have become
duplicated.  Tidy these up.  Rather than having the Assert check each
aspect of what makes a set invalid, here we introduce a helper function
which returns false when a set is invalid and have the Asserts use this
instead.

Also, make a pass on improving the comments in bitmapset.c.  Various
comments mentioned the input sets being "recycled".  This could be
interpreted to mean that the output set will always point to the same
memory as the given input parameter.  Here we try to make it clear that
this must not be relied upon and that callers must ensure that all
references to a given set are updated on each modification.

In passing, improve comments for bms_union(), bms_intersect() and
bms_difference() to detail what they do.  I (David) have too often had to
remind myself by reading the code each time to find out if I need, for
example, to use bms_union() or bms_join().  I also removed some
low-value comments that were trying to convey information about "these
operations" without mentioning which operations it was talking about.
It seems better to document these things in the function header comment
instead.

Author: Richard Guo, David Rowley
Discussion: https://postgr.es/m/CAMbWs4-djy9qYux2gZrtmxA0StrYXJjvB-oqLxn-d7J88t=PQQ@mail.gmail.com

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