Merge branch 'kh/doc-trailers' into seen

Documentation updates.

* kh/doc-trailers:
  doc: interpret-trailers: document comment line treatment
  doc: interpret-trailers: commit to “trailer block” term
  doc: interpret-trailers: add key format example
  doc: interpret-trailers: explain key format
  doc: interpret-trailers: explain the format after the intro
  doc: interpret-trailers: not just for commit messages
  doc: interpret-trailers: use “metadata” in Name as well
  doc: interpret-trailers: replace “lines” with “metadata”
  doc: interpret-trailers: stop fixating on RFC 822