doc: Improve description of wal_compression

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