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
DOC: fix doc build warnings
TST: GH6410 / numpy 4328
Merge pull request #6413 from jreback/32_bit_tests

TST: GH6410 / numpy 4328
BLD: add optional numpy_dev build
Merge pull request #6414 from jreback/numpy_dev

BLD: add optional numpy_dev build
BLD: tweak numpy_dev build
Merge pull request #6362 from jorisvandenbossche/doc-rolling

DOC: clarify docstring of rolling/expanding moments