DOC better wording in changelog legend

Author: jnothman

doc/whats_new.rst

@@ -11,15 +11,15 @@ Release notes for current and recent releases are detailed on this page, with
 **Tip:** `Subscribe to scikit-learn releases <https://libraries.io/pypi/scikit-learn>`__
 on libraries.io to be notified when new versions are released.
 
-Legend
-------
+Legend for changelogs
+---------------------
 
 - |MajorFeature|: something big that you couldn't do before.
 - |Feature|: something that you couldn't do before.
 - |Efficiency|: an existing feature now may not require as much computation or
   memory.
-- |Enhancement|: miscellaneous minor improvement.
-- |Fix|: something that didn't work as documentated before should now work.
+- |Enhancement|: a miscellaneous minor improvement.
+- |Fix|: something that previously didn't work as documentated should now work.
 - |API|: you will need to change your code to have the same effect in the
   future; or a feature will be removed in the future.