[docs] [issue13766] explain the relationship between Lib/lib2to3/Grammar.txt and Grammar/Grammar

[Éric Araujo]

Changes by Éric Araujo <merwok at netwok.org>:


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nosy: +benjamin.peterson, eric.araujo, gvanrossum

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13766>
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