Push options: print help information
### Problem to solve
While [push options](https://docs.gitlab.com/ee/user/project/push_options.html#push-options-for-merge-requests) is a very powerful feature, it can be [opaque and hard to use](https://news.ycombinator.com/item?id=31010933) even for folks who are [relatively versed](https://news.ycombinator.com/item?id=31021991) with GitLab and the command line.
### Proposal
Add a push option that doesn't actually complete the push, but instead sends back an explanation of using it to the command line like:
`git push -o gitlab.help`
or similar.
I'd guess returning like the 5 most common options and then the URL to the help page would be stellar
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issue