doc correction and refactoring for #275

Plugin-provided template fields are not available for queries. (Perhaps they
should be, though!)

beetsplug/missing.py

@@ -135,17 +135,19 @@ class MissingPlugin(BeetsPlugin):
                 print(sum([_missing_count(a) for a in albums]))
                 return
 
+            # Default format string for count mode.
+            if count and not fmt:
+                fmt = '$albumartist - $album: $missing'
+
             for album in albums:
                 if count:
                     missing = _missing_count(album)
                     if missing:
-                        if not fmt:
-                            fmt = '$albumartist - $album: $missing'
-                        print_obj(album, lib, fmt=fmt.format(missing))
-                    continue
+                        print_obj(album, lib, fmt=fmt)
 
-                for item in _missing(album):
-                    print_obj(item, lib, fmt=fmt)
+                else:
+                    for item in _missing(album):
+                        print_obj(item, lib, fmt=fmt)
 
         self._command.func = _miss
         return [self._command]

docs/plugins/missing.rst

@@ -60,12 +60,11 @@ total
 The ``total`` option (default: false) prints a single
 count of missing tracks in all albums
 
-Metadata fields
+Template Fields
 ---------------
 
-With this plugin enabled, templates can reference ``$missing`` as they
-can any standard metadata field for use in :doc:`Queries
-</reference/query>` and :doc:`Path Formats </reference/pathformat>`.
+With this plugin enabled, the ``$missing`` template field expands to the
+number of tracks missing from each album.
 
 Examples
 --------