Follow-up from "feat(update check): Add env variable to skip update check"

The following discussion from !1858 (merged) should be addressed:

• @faleksic started a discussion: (+5 comments)

  While this is useful when running glab in CI environments I am not sure where we can document this functionality, perhaps in docs/source/config/set.md and in commands/config/config.go?

  CC @theoretick