Add documentation for `dryrun` parameter (#2311)

The `dryrun` paramater's documentation is unhelpful (`dryrun mode`).

In the absense of proper documentation, [this definition from a comment on this topic](https://github.com/nektos/act/issues/268#issuecomment-1054834689) should be used.

Fixes: https://github.com/nektos/act/issues/268

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>