DOC: clarify docstring of rolling/expanding moments
- document center argument
- add note about the result set at right edge by default
- clarified freq keyword a little bit

Closes #5504

Two more things:
- there is some more room in avoiding duplication of the docstrings (more sharing)
- there seems to be a problem with the `@Appender` decorator picking up the docstring of the underlying cython function (see eg `expanding_median` at http://pandas.pydata.org/pandas-docs/dev/api.html#standard-expanding-window-functions)
 related is #4950
 yes I saw that now. The PRs are not overlapping, but that will be a more painfull rebase for one of us then .. (the other PR also solves a little bit the first point of avoiding duplication)
 Merging this as at the other PR there is not direct action.
