feat: Update API documentation on MR (auto_merge_strategy)

Problem to solve

Hello maintainers,

Could you please check if all the available options are documented in the MR API page at: https://docs.gitlab.com/ee/api/merge_requests.html

To be more specific, I was searching for way to have better automated merging pipeline and could not find a way until I stumbled upon: https://forum.gitlab.com/t/how-to-toggle-merge-when-pipeline-succeeds-via-gitlab-api/15065/8 talking about: auto_merge_strategy

Maybe I missed something but I would think it's missing on the page and should probably be added 😄

Proposal

Add the details about the options on the page 😄

Who can address the issue

Maybe @trakos (as I saw he submitted a previous commit on the page).

Other links/references

Thanks and good day!

Regards