Cross-link 3 similar API features

What does this MR do?

In #390454 (closed), specifically the Other links and references subheading, @bhaalsen provided some feedback I could tackle relatively easily. (That is not to say I will have gotten the phrasing right the first time.)

• Three APIs provide endpoints related to commenting on merge requests, and those pages are not crosslinked:
  • Commits API, specifically "Post comment to commit"
  • Notes API, specifically "Create new merge request note"
  • Discussions API, specifically "Create a new thread in the merge request diff"
• No portion of the Suggest Changes API page describes how to create a new suggestion. (Answer, in short: it's a specially formatted comment, but we should actually SAY that instead of leaving it for the user to guess.)
  • Subheadings are supposed to be in present tense, and not use gerunds. On such a short page, the different subheading formats would be obvious, so I renamed the other two subheads "Applying a suggestion" and "Applying multiple suggestions" for consistency with Create a suggestion.

Related issues

Related to API Documentation: Create multiple batched/draf... (#390454 - closed) which contains (helpful!) user feedback from @bhaalsen

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