pgsql: Add documentation mention that full page writes have to be

Log Message:
-----------
Add documentation mention that full page writes have to be enabled for
full pages to be restored during recovery.

Per private report from Michael Renner

Modified Files:
--------------
pgsql/doc/src/sgml:
wal.sgml (r1.57 -> r1.58)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/wal.sgml?r1=1.57&r2=1.58)