Remove ambiguity from merge request review docs

What does this MR do and why?

This is a follow-up on !207513.

This MR removes ambiguity from merge request review docs when it comes to mentioning pending comments and resolving threads.

It can confuse users into thinking that a resolve/reopen thread button does not act immediately when clicked, while in fact it acts immediately. Only quick actions are postponed that are typed directly in the comment itself.

To avoid this confusion we won't explicitly mention resolving threads, since a pending comment postpones all actions inside it until the review is submitted.