Documentation: get rid of write_super

The '->write_super' superblock method is gone, and this patch removes all the
references to 'write_super' from various pieces of the kernel documentation.

Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>