Document separator deprecation in lastgenre plugin

Addresses PR beetbox#6169 review comment about separator configuration.

Changes:
- Updated "Multiple Genres" section in lastgenre docs to explain that
  genres are now stored as a list and written as individual tags
- Added deprecation notice for the separator config option (deprecated
  in version 2.6) with explanation that it has no effect
- Added changelog entry documenting the separator deprecation

The separator option is now deprecated as genres are stored natively
as lists in the genres field and written to files as separate genre
tags rather than being joined with a separator.

Note: Migration logic to handle existing separator-joined genre strings
will be implemented as part of the database migration changes.

Author: jfot-cpu

docs/changelog.rst

@@ -111,6 +111,10 @@ Other changes:
   unavailable, enabling ``importorskip`` usage in pytest setup.
 - Finally removed gmusic plugin and all related code/docs as the Google Play
   Music service was shut down in 2020.
+- :doc:`plugins/lastgenre`: The ``separator`` configuration option is
+  deprecated. Genres are now stored as a list in the ``genres`` field and
+  written to files as individual genre tags. The separator option has no effect
+  and will be removed in a future version.
 
 2.5.1 (October 14, 2025)
 ------------------------

docs/plugins/lastgenre.rst

@@ -90,9 +90,8 @@ By default, the plugin chooses the most popular tag on Last.fm as a genre. If
 you prefer to use a *list* of popular genre tags, you can increase the number of
 the ``count`` config option.
 
-Lists of up to *count* genres will then be used instead of single genres. The
-genres are separated by commas by default, but you can change this with the
-``separator`` config option.
+Lists of up to *count* genres will be stored in the ``genres`` field as a list
+and written to your media files as separate genre tags.
 
 Last.fm_ provides a popularity factor, a.k.a. *weight*, for each tag ranging
 from 100 for the most popular tag down to 0 for the least popular. The plugin
@@ -192,7 +191,16 @@ file. The available options are:
   Default: ``no``.
 - **source**: Which entity to look up in Last.fm. Can be either ``artist``,
   ``album`` or ``track``. Default: ``album``.
-- **separator**: A separator for multiple genres. Default: ``', '``.
+- **separator**:
+
+  .. deprecated:: 2.6
+
+      The ``separator`` option is deprecated. Genres are now stored as a list in
+      the ``genres`` field and written to files as individual genre tags. This
+      option has no effect and will be removed in a future version.
+
+  Default: ``', '``.
+
 - **whitelist**: The filename of a custom genre list, ``yes`` to use the
   internal whitelist, or ``no`` to consider all genres valid. Default: ``yes``.
 - **title_case**: Convert the new tags to TitleCase before saving. Default: