Improve the documentation of E201/E202 (#14983)
## Summary
The summary is misleading, as well as the
`whitespace-after-open-bracket` and `whitespace-before-close-bracket`
names - it's not only brackets, but also parentheses and braces. Align
the documentation with the actual behaviour.
Don't change the names, but align the documentation with the behaviour.
## Test Plan
No test (documentation).
