TST: add comment that plotting tests are slow
- [ ] xref https://github.com/pandas-dev/pandas/pull/38079#issuecomment-735262800
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Add comment (module docstring) in ``pandas/tests/plotting/common.py``,
mentioning that all plotting tests are marked as slow.
thanks @ivanovmg 