DOC: Enhance asfreq docs
- [n/a] closes #xxxxx
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] tests added

Enhanced some docstrings for time-series related methods; mainly `asfreq`. Namely:
 - Converted `NDFrame.asfreq` into a template to use the specific `klass` in `Series.asfreq` and `DataFrame.asfreq`
 - Fully explain the behaviour of `NDFrame.asfreq` in the extended summary.
 - Add an extended summary to `asfreq` in `PeriodArray`/`PeriodIndex`
 - Use `klass` kwarg in docstring template for `PeriodAray.asfreq`\`PeriodIndex.asfreq`
 - Add an extended summary to `date_range`
 - Other very minor fixes/enhancements


The original motivator for this PR was [this question](https://stackoverflow.com/q/63860589/6117426), in particular the confusion of the asker.