doc: Improve description of wal_compression

The description of this GUC provides a list of the situations where
full-page writes are generated.  However, it is not completely exact,
mentioning only the cases where full_page_writes=on or base backups.  It
is possible to generate full-page writes in more situations than these
two, making the description confusing as it implies that no other cases
exist.

The description is slightly reworded to take into account that other
cases are possible, without mentioning them directly to minimize the
maintenance burden should FPWs be generated in more contexts in the
future.

Author: Jingtang Zhang <mrdrivingduck@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/CAPsk3_CtAYa_fy4p6=x7qtoutrdKvg1kGk46D5fsE=sMt2546g@mail.gmail.com
Backpatch-through: 13

doc/src/sgml/config.sgml

@@ -3169,8 +3169,9 @@ include_dir 'conf.d'
         This parameter enables compression of WAL using the specified
         compression method.
         When enabled, the <productname>PostgreSQL</productname>
-        server compresses full page images written to WAL when
-        <xref linkend="guc-full-page-writes"/> is on or during a base backup.
+        server compresses full page images written to WAL (e.g. when
+        <xref linkend="guc-full-page-writes"/> is on, during a base backup,
+        etc.).
         A compressed page image will be decompressed during WAL replay.
         The supported methods are <literal>pglz</literal>,
         <literal>lz4</literal> (if <productname>PostgreSQL</productname>