Add more explanation to the Python versions Status key

Author: nedbat

versions.rst

@@ -51,14 +51,24 @@ Full chart
 Status key
 ==========
 
-:feature: new features, bugfixes, and security fixes are accepted.
-:prerelease: feature fixes, bugfixes, and security fixes are accepted for the
-    upcoming feature release.
-:bugfix: bugfixes and security fixes are accepted, new binaries are still
-    released. (Also called **maintenance** mode or **stable** release)
-:security: only security fixes are accepted and no more binaries are released,
-    but new source-only versions can be released
-:end-of-life: release cycle is frozen; no further changes can be pushed to it.
+Python releases go through five phases:
+
+:feature: Before the first beta, the next full release can accept new features,
+   bug fixes, and security fixes.
+
+:prerelease: After the first beta, no new features can go in, but feature
+   fixes, bug fixes, and security fixes are accepted for the upcoming feature
+   release.
+
+:bugfix: Once a version has been released, bug fixes and security fixes are
+   accepted. New binaries are built and released. (Also called **maintenance**
+   mode or **stable** release)
+
+:security: After two years, only security fixes are accepted and no more
+   binaries are released.  New source-only versions can be released as needed.
+
+:end-of-life: Five years after a release, support ends. The release cycle is
+   frozen; no further changes can be pushed to it.
 
 See also the :ref:`devcycle` page for more information about branches and backporting.