Add a wiki section about the sniff documentation feature

[jrfnl]

Is your feature request related to a problem?

PHPCS can display documentation about sniffs using the --generator=... CLI flag.

Aside from in the --help section on the Usage page in the wiki, this feature isn't documented.

Describe the solution you'd like

Either a new section in the Advanced Usage page or a separate new Wiki page documenting this feature.

The docs should (at the very least) include information about what docs will display when (effect of --standard flag and of not using the flag, effect of --sniffs flag) and about the documentation formats available.

Additional context

Might also be good to see if - once the section/page is written up -, there are references in the pre-existing wiki docs which can start linking to the new docs. (like the "Requirements" page)

• I intend to create a pull request to implement this feature.

[rodrigoprimo]

@jrfnl, I prepared a commit to address this issue, but I'm unsure where to place it.

Either a new section in the Advanced Usage page or a separate new Wiki page documenting this feature.

I wonder whether the instructions for using the --generator argument should be placed on the Usage page instead. Learning more about what sniffs do feels like basic usage rather than advanced usage to me. Thoughts?