discuss how to use -p

Doc/library/2to3.rst

@@ -86,6 +86,14 @@ document could also be refactored with this option.
 The :option:`-v` option enables output of more information on the translation
 process.
 
+Since some print statements can be parsed as function calls or statements, 2to3
+cannot always read files containing the print function.  When 2to3 detects the
+presence of the ``from __future__ import print_function`` compiler directive, it
+modifies its internal grammar to interpert :func:`print` as a function.  This
+change can also be enabled manually with the :option:`-p` flag.  Use
+:option:`-p` to run fixers on code that already has had its print statements
+converted.
+
 
 .. _2to3-fixers: