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

[Roundup Robot]

Roundup Robot added the comment:

New changeset 871278b87c62 by Eli Bendersky in branch '3.4':
Issue #20375: Clarify ET's parsing of comments and processing instructions.
http://hg.python.org/cpython/rev/871278b87c62

New changeset 5c3166ec80e1 by Eli Bendersky in branch 'default':
Issue #20375: Clarify ET's parsing of comments and processing instructions.
http://hg.python.org/cpython/rev/5c3166ec80e1

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nosy: +python-dev

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