docs/plugins/discogs.rst: Update plugin documentation
Add Configuration section and describe the 'source_weight' option.
@@ -43,6 +43,13 @@ documentation), login to `Discogs`_, and visit the
token`` button, and place the generated token in your configuration, as the
``user_token`` config option in the ``discogs`` section.
+Configuration
+-------------
+
+- **source_weight**: Penalty applied to Discogs matches during import. Set to
+ 0.0 to disable.
+ Default: ``0.5``.
+
Troubleshooting
---------------
