docs(start): Document subscription override mode

It's debatable whether this belongs here. Having moved onto my own custom presets, the
only use cases for override mode in my subscriptions file are for those series that also
have TVDB metadata. Those ytdl-sub downloads I integrate into my Sonarr library need
per-subscription overrides such as:

- Sonarr manages years 2020-2023 for one series and ytdl-sub fills in
  from there, so set a `sonarr_series_after: "20240101"` override

- each series needs a directory named per Sonarr's configuration with the TVDB ID, the subscription key,
  but also a series name prefix without the TVDB ID for each episode file, so set a
  `sonarr_series_prefix: "Foo Series (2020)"` override

So are there other use cases for override mode that are more common and less niche than
mine?

docs/source/guides/getting_started/subscriptions.rst

@@ -142,6 +142,24 @@ the second ``= ...`` value specifies the rating for all descendant subscriptions
       "Foo Kids Band": "https://soundcloud.com/foo-kids-band"
 
 
+Override variables for one subscription
+---------------------------------------
+
+Most variable overrides aren't actually specific to just one subscription and should be
+set in :doc:`your own custom presets <./first_config>`. But use :ref:`the override mode
+'~...' prefix <config_reference/subscription_yaml:override mode>` when an override is
+specific to only one subscription and will never be shared with another:
+
+.. code-block:: yaml
+  :caption: subscriptions.yaml
+  :emphasize-lines: 2-
+
+  Jellyfin TV Show by Date:
+    "~NOVA PBS":
+      url: "https://www.youtube.com/@novapbs"
+      tv_show_directory: "/media/Unique/Series/Path"
+
+
 Next Steps
 ----------