DOC Describe what's new categories (scikit-learn#13697)

Author: jnothman

doc/whats_new.rst

@@ -11,6 +11,18 @@ 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
+------
+
+- |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.
+- |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.
+
 .. include:: whats_new/v0.21.rst
 .. include:: whats_new/v0.20.rst
 

doc/whats_new/v0.21.rst

@@ -51,12 +51,15 @@ Changelog
 Support for Python 3.4 and below has been officially dropped.
 
 ..
-    See version doc/whats_new/v0.20.rst for structure. Entries should be
-    prefixed with one of the labels: |MajorFeature|, |Feature|, |Efficiency|,
-    |Enhancement|, |Fix| or |API|. They should be under a heading for the
-    relevant module (or *Multiple Modules* or *Miscellaneous*), and within each
-    section should be ordered according to the label ordering above. Entries
-    should end with: :issue:`123456` by :user:`Joe Bloggs <joeongithub>`.
+    Entries should be grouped by module (in alphabetic order) and prefixed with
+    one of the labels: |MajorFeature|, |Feature|, |Efficiency|, |Enhancement|,
+    |Fix| or |API| (see whats_new.rst for descriptions).
+    Entries should be ordered by those labels (e.g. |Fix| after |Efficiency|).
+    Changes not specific to a module should be listed under *Multiple Modules*
+    or *Miscellaneous*.
+    Entries should end with:
+    :issue:`123456` by :user:`Joe Bloggs <joeongithub>`.
+    where 123456 is the *pull request* number, not the issue number.
 
 :mod:`sklearn.base`
 ...................