DOC: Clarify display.precision
- [x] closes #21004
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Thanks @jreback, @rhshadrach, updated. may I get this PR labelled as `hacktoberfest-accepted`?

See #36837 and [here](https://hacktoberfest.digitalocean.com/hacktoberfest-update). all checks passing.  thanks @hongshaoyang 