Added the "provisional package" glossary term from PEP 411

Doc/glossary.rst

@@ -523,6 +523,20 @@ Glossary
       definition), or pass several arguments as a list to a function.  See
       :term:`argument`.
 
+   provisional package
+      A provisional package is one which has been deliberately excluded from the
+      standard library's backwards compatibility guarantees.  While major
+      changes to such packages are not expected, as long as they are marked
+      provisional, backwards incompatible changes (up to and including removal
+      of the package) may occur if deemed necessary by core developers.  Such
+      changes will not be made gratuitously -- they will occur only if serious
+      flaws are uncovered that were missed prior to the inclusion of the
+      package.
+
+      This process allows the standard library to continue to evolve over time,
+      without locking in problematic design errors for extended periods of time.
+      See :pep:`411` for more details.
+
    Python 3000
       Nickname for the Python 3.x release line (coined long ago when the release
       of version 3 was something in the distant future.)  This is also