Fixed more SS03 errors
Fixed SS03 errors in the docstrings for `pandas.tseries.frequencies.to_offset`; `pandas.Int64Index`; `pandas.UInt64Index`; `pandas.Float64Index`; `pandas.IntervalIndex`; `pandas.plotting.plot_params`; `pandas.io.formats.style.Styler.loader`.

- [x] xref #29315
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This is part of the docstring errors in #27977.

thanks @SebStrug 
does this close *all* of #29315 ? @jreback 
Two still remain:
- [ ] pandas.Timestamp.isoweekday: Summary does not end with a period
- [ ] pandas.Timestamp.weekday: Summary does not end with a period
I couldn't find these in the codebase.

More SS03 errors exist for `pandas.tseries.offsets` but these are false-positives according to @datapythonista. I haven't investigated this, but could open an issue after I've looked into it. We had some cases where the validation script was validating a Python function we were wrapping, and not our function. That's probably the case for `isoweekday` and `weekday`. I think the new version of the script in numpydoc should fix that problem, so probably better to forget about them for now, and when we use the new version of the script, we'll see what's pending.