[docs] [issue18389] document that os.path.relpath does not interrogate the file system

[Roundup Robot]

Roundup Robot added the comment:

New changeset 70837970c5d8 by R David Murray in branch '3.3':
#18389: Clarify that relpath does not access the file system.
http://hg.python.org/cpython/rev/70837970c5d8

New changeset 7de05609e390 by R David Murray in branch 'default':
#18389: Clarify that relpath does not access the file system.
http://hg.python.org/cpython/rev/7de05609e390

New changeset 1345d8dbcb19 by R David Murray in branch '2.7':
#18389: Clarify that relpath does not access the file system.
http://hg.python.org/cpython/rev/1345d8dbcb19

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nosy: +python-dev

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<http://bugs.python.org/issue18389>
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