Clarify subheadline of maintenance policy

Not all releases are using `X.Y.Z`.

guides/source/maintenance_policy.md

@@ -5,7 +5,7 @@ Maintenance Policy for Ruby on Rails
 
 Support of the Rails framework is divided into four groups: New features, bug
 fixes, security issues, and severe security issues. They are handled as
-follows, all versions in `X.Y.Z` format.
+follows, all versions, except for security releases, in `X.Y.Z`, format.
 
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