Move UserString to collections.
Removed decode() method. Added isidentifier() and format() methods.
Drop MutableUserString class.
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index bb2140c..1d6687d 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -706,3 +706,22 @@
special methods supported by this class will need to be overridden; please
consult the sources for information about the methods which need to be provided
in that case.
+
+:class:`UserString` objects
+-------------------------
+
+The class, :class:`UserString` acts as a wrapper around string objects.
+The need for this class has been partially supplanted by the ability to
+subclass directly from :class:`str`; however, this class can be easier
+to work with because the underlying string is accessible as an
+attribute.
+
+.. class:: UserString([sequence])
+
+ Class that simulates a string or a Unicode string object. The instance's
+ content is kept in a regular string object, which is accessible via the
+ :attr:`data` attribute of :class:`UserString` instances. The instance's
+ contents are initially set to a copy of *sequence*. The *sequence* can
+ be an instance of :class:`bytes`, :class:`str`, :class:`UserString` (or a
+ subclass) or an arbitrary sequence which can be converted into a string using
+ the built-in :func:`str` function.