Use 'multiple-use' terminology for Default-Extra metadata field, and mention that Metadata-Version will need to be bumped

Author: astrofrog

peps/pep-0771.rst

@@ -91,27 +91,32 @@ Specification
 ``Default-Extra`` Metadata Field
 ---------------------------------
 
-A new metadata field, ``Default-Extra``, will be added to the `core package
+A new multiple-use metadata field, ``Default-Extra``, will be added to the `core package
 metadata <https://packaging.python.org/en/latest/specifications/core-metadata/#core-metadata>`_.
-This field allows package maintainers to define an extra that is
-automatically included when a user installs the package without specifying any
-extras explicitly::
+For this field, each entry must be a string specifying an extra that will be
+automatically included when the package is installed without any extras specified explicitly.
 
-    Default-Extra: recommended
+Only entries already specified in a `Provides-Extra
+<https://packaging.python.org/en/latest/specifications/core-metadata/#provides-extra-multiple-use>`_
+entry can be used in a ``Default-Extra`` entry.
 
-If multiple default extras are needed, one ``Default-Extra:`` entry
-must be provided for each one::
+Examples::
 
+    Default-Extra: recommended
     Default-Extra: backend1
     Default-Extra: backend2
     Default-Extra: backend3
 
+Since this introduces a new field in the core package metadata, this will require
+`Metadata-Version <https://packaging.python.org/en/latest/specifications/core-metadata/#metadata-version>`_
+to be bumped to the next minor version (2.5 at the time of writing).
+
 New key in ``[project]`` metadata table
 ---------------------------------------
 
 A new key will be added to the ``[project]`` table in project metadata as
 originally defined in :pep:`621` and now defined in the `PyPA specifications
-<https://packaging.python.org/en/latest/specifications/>`_. This key will be named
+<https://packaging.python.org/en/latest/specifications/pyproject-toml/>`_. This key will be named
 ``default-optional-dependencies`` with the following description:
 
 * `TOML <https://toml.io/>`_ type: Array of strings