bug#11899: 24.1.50; Weird names and unhelpful docstrings for some python function

[Chong Yidong]

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:

>>   python-nav-sentence-start
>>   python-nav-sentence-end
>>   python-nav-forward-sentence
>>   python-nav-backward-sentence
>> have weird names in the sense that they don't move by sentences.
>
> Indeed, thanks for catching that.  Fabián, can you take a look at it?

For consistency with the rest of the Emacs code base, I suggest dropping
the `nav', since other modes don't use that naming convention.  As for
the bug report, I don't have an opinion on whether it should be
`python-sentence-start' or `python-statement-start'.  If the former, it
would be nice if the docstring explains what "sentence" means in the
Python context.