Merge #16690: Prepare release notes for 0.19

74e38769895c643d75f77634519c1442fa38bad6 Release notes: add previously undocumented changes (David A. Harding)
7e1634a927f86dcffc7e35fdd479d19c414b14ab Release notes: edit previously-detached notes (David A. Harding)
e7415a5a95f53abf1f8ffa2085217b249d7caf61 Doc: move detached release notes into release-notes.md (David A. Harding)

Pull request description:

  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.

ACKs for top commit:
  fanquake:
    ACK 74e38769895c643d75f77634519c1442fa38bad6 - Thanks for doing this. It's nice to have a gauge of what's going to be in the `0.19.0` release.
  meshcollider:
    ACK 74e38769895c643d75f77634519c1442fa38bad6

Tree-SHA512: 676668765849d5a67520dd8ac49de85ac1bfb5ba2dc09504e75db77d79c7e2c58b5cee16c58591ec575cb3682e630231baba7fd07565d19f8d02243e06fcb9ab

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