pgsql: Update write-cache docs to mention windows behavior for various

Log Message:
-----------
Update write-cache docs to mention windows behavior for various
fsync_methods.

Magnus.

Modified Files:
--------------
pgsql/doc/src/sgml:
wal.sgml (r1.49 -> r1.50)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/wal.sgml?r1=1.49&r2=1.50)