[docs] [issue20375] ElementTree: Document handling processing instructions

[Nikolaus Rath]

Nikolaus Rath added the comment:

Thanks for the commit!

My intention is to fix the behavior itself for 3.5 (see issue 9521), so I think adding testcases for the old behavior in the meantime isn't necessary.

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<http://bugs.python.org/issue20375>
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