Documentation: move mandatory locking documentation to filesystems/

Shouldn't this mandatory-locking documentation be in the
Documentation/filesystems directory?

Give it a more descriptive name while we're at it, and update 00-INDEX
with a more inclusive description of Documentation/filesystems (which
has already talked about more than just individual filesystems).

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>