FIXME: A first version of new lastgenre docs

force configuration description is not entirely correct yet...

docs/plugins/lastgenre.rst

@@ -111,6 +111,22 @@ Last.fm returns both of those tags, lastgenre is going to use the most
 popular, which is often the most generic (in this case ``folk``). By setting
 ``prefer_specific`` to true, lastgenre would use ``americana`` instead.
 
+Handling pre-populated tags
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+The ``force`` and ``keep_allowed`` options control how pre-existing genres are
+handled. By default, the plugin fetches new genres, even if the files come with
+pre-populated genre tags (``force: yes``) and at the same it makes sure
+whitelisted genres stay in-tact (``keep_allowed: yes``).
+
+To overwrite any content of pre-populated tags, set ``force: yes`` and
+``keep_allowed: no``.
+
+To only write new genres to empty tags and keep all pre-existing tag content
+as-is, set ``force: no`` and ``keep_allowed: no``.
+
+To only write new genres to empty tags and make sure only whitelisted genres
+remain in pre-populated tags, set ``force: no`` and ``keep_allowed: yes``.
+
 Configuration
 -------------
 
@@ -129,8 +145,12 @@ configuration file. The available options are:
   You can use the empty string ``''`` to reset the genre.
   Default: None.
 - **force**: By default, beets will always fetch new genres, even if the files
-  already have one. To instead leave genres in place in when they pass the
-  whitelist, set the ``force`` option to ``no``.
+  already have one. To leave existing genres in place, set the ``force``
+  option to ``no`` and set the ``keep_allowed`` option to your preference.
+  Default: ``yes``.
+- **keep_allowed**: By default, only whitelisted genres remain in pre-populated
+  tags. Set the ``keep_allowed`` option to ``no`` to keep any content in
+  pre-populated tags.
   Default: ``yes``.
 - **min_weight**: Minimum popularity factor below which genres are discarded.
   Default: 10.