Clarify old WAL files cannot be removed until they are summarized

Hi,

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. Attached patch adds that clarification to
the documentation. Thought?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION