Update period.py

proposed fix for #4699: Period() docstring inconsistent with code when freq has a mult != 1
DOC: added "Must be a singlular rule-code (e.g. 5T is not allowed)."
BUG: fix DatetimeIndex join with PeriodIndex stack overflow
TST: add test for joining PeriodIndex with DatetimeIndex
CLN: remove unused imports
CLN: remove copying of PeriodIndex values on astype(int)
Merge pull request #5101 from cpcloud/fix-period-datetime-join-stack-overflow

BUG: fix PeriodIndex join with DatetimeIndex stack overflow
Merge pull request #5057 from JackKelly/master

DOC: proposed fix for #4699: Period() docstring inconsistent with code when freq has a mult != 1
CLN: tiny typo from GH5057: Period() docstring
BUG: allow plot, boxplot, hist and completion on GroupBy objects
BUG: catch attribute error for raw ndarrays to avoid duplicate plots
DOC: add frame tab completion example in 10min.rst
DOC: doc fixups
Merge pull request #5111 from cpcloud/infer-dist-doc-build-failure

DOC: doc fixups
Merge pull request #5105 from cpcloud/enable-groupby-plot-and-completion

BUG: allow plot, boxplot, hist and completion on GroupBy objects
API: convert objects on fillna when object result dtype, related (GH5103)