Further document formatted modify with examples

I think these can make it clearer why someone would want to use this
feature. (Part of beetbox#4095.)

docs/changelog.rst

@@ -137,6 +137,8 @@ Major new features:
   Thanks to :user:`edgars-supe`.
 * The :ref:`modify-cmd` and :ref:`import-cmd` can now use
   :doc:`/reference/pathformat` formats when setting fields.
+  For example, you can now do ``beet modify title='$track $title'`` to put
+  track numbers into songs' titles.
   :bug:`488`
 
 Other new things:

docs/reference/cli.rst

@@ -267,8 +267,12 @@ artist="Tom Tom Club"`` will change the artist for the track "Genius of Love."
 To remove fields (which is only possible for flexible attributes), follow a
 field name with an exclamation point: ``field!``.
 
-Values support the same template syntax as beets'
+Values can also be *templates*, using the same syntax as
 :doc:`path formats <pathformat>`.
+For example, ``beet modify artist='$artist_sort'`` will copy the artist sort
+name into the artist field for all your tracks,
+and ``beet modify title='$track $title'`` will add track numbers to their
+title metadata.
 
 The ``-a`` switch also operates on albums in addition to the individual tracks.
 Without this flag, the command will only change *track-level* data, even if all