Documentation: remove outdated information from automount-support.txt

The guidelines for adding automount support to a filesystem
in filesystems/automount-support.txt is out or date.
filesystems/autofs4.txt contains more current text, so replace
the out-of-date content with a reference to that.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>