Add docs for mod -a --noinherit option

and further clarify `mod -a` docs:
Even though e39dcfc and the linked discussion
already does a very good job on clarifying what is actually happening when `mod
-a` is issued, this commit adds further details about the difference between
the album query and what is actually modified.

docs/reference/cli.rst

@@ -257,7 +257,7 @@ modify
 ``````
 ::
 
-    beet modify [-MWay] [-f FORMAT] QUERY [FIELD=VALUE...] [FIELD!...]
+    beet modify [-IMWay] [-f FORMAT] QUERY [FIELD=VALUE...] [FIELD!...]
 
 Change the metadata for items or albums in the database.
 
@@ -274,13 +274,17 @@ name into the artist field for all your tracks,
 and ``beet modify title='$track $title'`` will add track numbers to their
 title metadata.
 
-The ``-a`` switch also operates on albums in addition to the individual tracks.
+The ``-a`` option changes to querying album fields instead of track fields and
+also enables to operate on albums in addition to the individual tracks.
 Without this flag, the command will only change *track-level* data, even if all
 the tracks belong to the same album. If you want to change an *album-level*
 field, such as ``year`` or ``albumartist``, you'll want to use the ``-a`` flag
 to avoid a confusing situation where the data for individual tracks conflicts
 with the data for the whole album.
 
+Modifications issued using ``-a`` by default cascade to individual tracks. To
+prevent this behaviour, ``-I/--noinherit`` can be used.
+
 Items will automatically be moved around when necessary if they're in your
 library directory, but you can disable that with  ``-M``. Tags will be written
 to the files according to the settings you have for imports, but these can be