DOC: autosummary table summary lines (sphinx 1.2.1)
Related to #5899.

With the release of Sphinx 1.2.1, there is also a change in the behaviour of `sphinx.ext.autosummary` in how to define the text shown in the autosummary table (the tables you see here: http://pandas.pydata.org/pandas-docs/dev/api.html).

See:
- sphinx issue: https://bitbucket.org/birkenfeld/sphinx/issue/943/autosummary-doesnt-always-get-first
- http://stackoverflow.com/questions/10718846/use-first-paragraph-instead-of-first-line-in-sphinx-autosummary

**Summary**: the regex to define the part (idea: the first sentence) of the docstring that is used as the summary line in the tables has changed to allow the first sentence to be longer than the first line.

**Consequence for pandas**: The new regex searches for the end of the first sentence on all lines. In case of pandas docstrings, which are not that cleaned up to always have a period `.` at the end of the first line, this will sometimes give a first sentence including part of the parameter explanations. 
Result: messy table in api.html

**To do**: go over all docstrings of public functions to adapt first sentence. 
They should look like this:

```
Short summary line.

Optionally longer explanation.
bla bla

Parameters
----------
...
```

where especially the period `.` after the first short line is important.
