Cover counting option multiplicity in the manual

[f4lco]

The section on element... does not cover that the syntax is also capable of the number of repetitions of a certain flag (see this SO answer).

This is immensely useful for some flags. Take verbosity as an example: we would pass -v to a script, such that its output include some diagnostic messages, and -vv (very verbose) would make the output a full debug log.

I think the website should cover this use case, and I would be happy to file a PR if wanted.