Indicate how FILES are checked out.
If FILES are not registered, this function always returns nil.
For registered files, the possible values are:
'implicit FILES are always writable, and checked out `implicitly'
when the user saves the first changes to the file.
'locking FILES are read-only if up-to-date; user must type
C-x v v before editing. Strict locking
is assumed.
'announce FILES are read-only if up-to-date; user must type
C-x v v before editing. But other users
may be editing at the same time.