[docs] [issue8996] Add a default role to allow writing bare `len` instead of :func:`len`

[anatoly techtonik]

anatoly techtonik <techtonik at gmail.com> added the comment:

`someref` would be more convenient than :func:`someref` but in case of namespace conflict (for example, as a result of documentation merge it both :func:`someref` and :class:`someref` can appear) - it would be necessary to go through all the links in both sources and update ambiguous references to point to the right type.

But perhaps the biggest problem would be to explain users what happens. Also in the case when you create new :class:`someref` and Sphinx detects conflict.

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nosy: +techtonik

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue8996>
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