doc: Clarify old WAL files are kept until they are summarized.

doc: Clarify old WAL files are kept until they are summarized.

The documentation in wal.sgml explains that old WAL files cannot be
removed or recycled until they are archived (when WAL archiving is used)
or replicated (when using replication slots). However, it did not mention
that, similarly, old WAL files are also kept until they are summarized
if WAL summarization is enabled. This commit adds that clarification
to the documentation.

Back-patch to v17 where WAL summarization was added.

Author: Fujii Masao
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/fd0eb0a5-f43b-4e06-b450-cbca011b6cff@oss.nttdata.com