DOC: clean up some doc-build warnings
only big ones left are these

```
/Users/jreback/pandas/doc/source/generated/pandas.Series.str.partition.rst:6: WARNING: don't know which module to import for autodocumenting u'Series.str.partition' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
/Users/jreback/pandas/doc/source/generated/pandas.Series.str.repeat.rst:6: WARNING: don't know which module to import for autodocumenting u'Series.str.repeat' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
```

and one on `groupby.nth` I can't seem to figure out...
 On the `groupby.nth`, I also already looked at before, but didn't spot the error either
 merged via https://github.com/pydata/pandas/commit/fcab956d41ce74a927f30b3723f0fdfde7df75b6
