DOC: remove docstring of flags attribute during doc building process (#5331)
DOC: small error in cookbook (doc build warning)
BUG: Excel writer doesn't handle "cols" option correctly

Added fix to ensure that Series associated with column
names are handled correctly even if names are duplicated
or swapped. Issue #5427
API: raise SettingWithCopy when chained assignment is detected
BUG: pd.to-timedelta handles missing data
PERF: Use xlsxwriter by default (if available)
PERF: performance improvements on isnull/notnull for large pandas objects
DOC: add basic documentation to some of the GroupBy properties and methods
DOC: add GroupBy class to api.rst
DOC: add import for GroupBy object to api.rst
DOC: add GroupBy.aggregate and GroupBy.transform to api.rst
DOC: add meta comment to api.rst about when GroupBy objects are created