DOC: styling clean-up of docstrings (part 1, frame.py)

Some stylistic clean-up of the docstrings in frame.py.

Things changed are mainly:
- missing spaces aroung the `:` in `param : type`
- explanation of param on second line (instead of on the same line -> there should be the type info)
- add bullets to enumerations of possible values of a keyword (so they are rendered as a list in the html docstring instead of one continuous block of text) Eg the `method` keyword explanation in http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.corr.html