9.3: mention checksum feature in "29.1. Reliability"

This page:

  http://www.postgresql.org/docs/9.3/static/wal-reliability.html

says:

"Data pages are not currently checksummed, though full page images
recorded in WAL records will be protected. Data pages have a 16-bit
field available for future use with a data page checksum feature."

Patch with correction suggestion attached.

Regards

Ian Barwick