doc/protocol.rst: "list file" is deprecated

NEWS

@@ -10,6 +10,7 @@ ver 0.21.8 (not yet released)
 * fix build failure with GCC 9
 * fix build failure with -Ddatabase=false
 * systemd: add user socket unit
+* doc: "list file" is deprecated
 
 ver 0.21.7 (2019/04/03)
 * input

doc/protocol.rst

@@ -861,8 +861,7 @@ The music database
 :command:`list {TYPE} {FILTER} [group {GROUPTYPE}]`
     Lists unique tags values of the specified type.
     ``TYPE`` can be any tag supported by
-    :program:`MPD` or
-    *file*.
+    :program:`MPD`.
 
     Additional arguments may specify a :ref:`filter <filter_syntax>`.
     The *group* keyword may be used
@@ -873,6 +872,10 @@ The music database
 
      list album group albumartist
 
+    ``list file`` was implemented in an early :program:`MPD` version,
+    but does not appear to make a lot of sense.  It still works (to
+    avoid breaking compatibility), but is deprecated.
+
 .. _command_listall:
 
 :command:`listall [URI]`