Add missing docs: reviewing one file at a time

Problem to solve, small version

Simple version: we lack documentation on reviewing one file at a time, as Kai noted in Add missing docs: file tree (#423475).

I'm not sure I know everything I need to know about this process, because I haven't done it.

Problem to solve, large version

(It may or may not be appropriate to tackle this larger problem at this time.)

@phikai and I have been aware for some time that https://docs.gitlab.com/ee/user/project/merge_requests/reviews/ combines the concept of "reviews" (a thing that a merge request undergoes) and "reviewers" (a related but separate concept). I don't know if we should try to tackle that as part of documenting this smaller feature, but in good conscience, I have to at least mention it.

Other links/references

• Related to Features missing documentation in Code Review (#371903 - closed) where the need was identified
• Related to Add missing docs: file tree (#423475) which is the other docs lack Kai identified