pgsql: postgres_fdw: Adjust warning message in statistics import.

postgres_fdw: Adjust warning message in statistics import.

The previous warning message when falling back to sampling in a certain
case failed to properly mention the reason for that.

Also, for clarity, rewrite a comment about the fallback and add a note
about it to postgres-fdw.sgml.

Follow-up to commit 6a8571568.

Reported-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwHq1VuXwxeDU8N4dd9U4RFEW982qQ9C26mRQDs3nu_Naw%40mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9f4bd91a1960f0263c497a9a1ddf121cbcefbb98

Modified Files
--------------
contrib/postgres_fdw/postgres_fdw.c | 10 +++++-----
doc/src/sgml/postgres-fdw.sgml | 6 ++++++
2 files changed, 11 insertions(+), 5 deletions(-)