Doc: extend warnings about collation-mismatch hazards in postgres_fdw.

Doc: extend warnings about collation-mismatch hazards in postgres_fdw.

Be a little more vocal about the risks of remote collations not
matching local ones.  Actually fixing these risks seems hard,
and I've given up on the idea that it might be back-patchable.
So the best we can do for the back branches is add documentation.

Per discussion of bug #16583 from Jiří Fejfar.

Discussion: https://postgr.es/m/2438715.1632510693@sss.pgh.pa.us