Update git operation architecture docs

What does this MR do?

Replaces some TODOs with a bit of detail on how git operations over HTTP and SSH proceed. Asked about in Slack by @francispotter : https://gitlab.slack.com/archives/CK75EF2A2/p1584382753170300

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content