Some additional doc changes based around compression of page images in

Some additional doc changes based around compression of page images in
WAL and the interaction of the new full_page_writes parameter with PITR.

The too-small WAL first sect1 has been merged with the one following
sect1 for clarity.

Some minor comments have been made in the WAL config section also.

Passes SGML make and proofread for typos.
Files changed:
patching file doc/src/sgml/backup.sgml
patching file doc/src/sgml/config.sgml
patching file doc/src/sgml/wal.sgml

Simon Riggs