Add a comment how the fix_imports fixer is an example of how to get imports

changed by 2to3.

pep-3108.txt

@@ -912,7 +912,8 @@ will be accomplished by using the ``as`` keyword in import statements
 to bind in the module namespace to the old name while importing based
 on the new name (when the keyword is not already used, otherwise the
 re-assigned name should be left alone and only the module that is
-imported needs to be changed).
+imported needs to be changed). The ``fix_imports`` fixer is an
+example of how to approach this.
 
 
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