DOC : Added docstrings to _CollectionWithSizes

Added docstrings for get_sizes and set_sizes
DOC : tweaked Collection api auto-doc properties

Include inherited methods (so get_sizes and set_sizes
show up) in auto-doc
DOC : added whats_new entry for _CollectionWithSizes
Merge pull request #3130 from tacaswell/scatter_set_sizes_whats_new

Scatter set sizes whats new