Merges in detached release notes, edits each change down to a single paragraph bullet point (or, in a couple cases, two individual bullet points in separate sections each with a single paragraph). Adds notes for some undocumented changes I found reviewing git log --merges. Also tries something new: adds the PR number(s) after each entry to make it easier for both reviewers and end-user readers to look up the details behind each change. (If the PR numbers are unwanted, they're easy to remove either in this PR or later in the release process.)
I also checked the 0.18 branch but I didn't find anything in the current release notes that had been backported.
A particular focus in my editing was trying to keep things concise, particularly by pointing to RPC documentation when available (or upcoming, as in #16629). I do suspect that one downside of detached notes is that people write longer summaries than they would if they knew there were already 300 other lines of release notes. :-)
The first commit only moves notes, puts them in bullet form, adjusts indentation appropriately, and drops unneeded headers. It can be reviewed with git diff --color-moved=dimmed-zebra for a little bit of a speedup, but unfortunately I wasn't smart enough to split my copy/pasting and line wrapping into separate commits, so it's not a transparently move-only change.
