Fixes small error in the example at the top of the article, and
adds a short statement about narrows searching the current user's
personal message history with a link to the help center article.
Gives the final section a subheader and adds the two other narrow
operators that take an ID.
These docs had a number of typos, and also didn't fully clarify how to
handle lists of integers for `pm-with`.
Also makes some tweaks to the user docs to make this clearer as well.
Fixes#13167.
We also document support for user IDs in the pm-with narrow operator.
Edited by tabbott to document on /api rather than in the /help page.
Fixes part of #9474.
Lauryn Menard
Anders Kaseorg
m-e-l-u-h-a-n
Tim Abbott
Priyank Patel
Yago González