Clarify comments about sanitized_allowed_tags

- Clarify that arguments are expected in array format.
- Extension of https://github.com/rails/rails/pull/17390.
- https://github.com/rails/rails/pull/17390 was targeted against
  4-1-stable branch. This commit updates master.
- [ci skip]
Merge pull request #17392 from prathamesh-sonpatki/fix-docstring-for-sanitized-helpers-ci-skip

Clarify comments about sanitized_allowed_tags