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.fujii@gmail.com>
Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwHq1VuXwxeDU8N4dd9U4RFEW982qQ9C26mRQDs3nu_Naw%40mail.gmail.com
Backpatch-through: 19

doc/src/sgml/postgres-fdw.sgml

@@ -389,6 +389,12 @@ OPTIONS (ADD password_required 'false');
        table would have attempted to fetch and import the remote statistics
        first if this option was enabled.
       </para>
+
+      <para>
+       Note that this option doesn't currently support the case where the
+       remote table is (or was once) inherited.  This is an implementation
+       restriction that might be fixed in a future release.
+      </para>
      </listitem>
     </varlistentry>