Add section about ghost pull requests. - #14580

@sampablokuper my text in #13887 (comment) was pretty carefully crafted. Any reason for the change here?

@Hainish, yes. Your text is good, but:

• Your text implicitly raised, yet did not explicitly answer, the question, "Don't maintainers reserve the right to close PRs without any comment, regardless?"
• The first sentence of your second paragraph seemed superfluous. Having already explained, by that point, what @ghost PRs are and how they arise, you do not need to also tell readers how to avoid creating them, because it is implicit in what you have already said.

My wording attempts to address those points, and also to be more concise. Running wc on our respective texts gives:

$ wc hainish.md
  3 102 775 hainish.md
$ wc spk.md
  1  52 429 spk.md

i.e. a ~49% reduction in words and a ~45% reduction in characters.

If you still prefer your wording, I am happy to modify this PR accordingly 🙂

@sampablokuper I do prefer my wording, making specific why it is inconvenient for maintainers. I've committed this in 376c84a. Thanks for this though!