Merge branch 'kh/doc-trailers' into jch

Documentation for 'git interpret-trailers' has been updated to explain
the format of trailer keys (alphanumeric characters and hyphens),
replace outdated terminology, define key terms upfront, and document
how comment lines in the input are treated.

* kh/doc-trailers:
  doc: interpret-trailers: document comment line treatment
  doc: interpret-trailers: rewrite new-trailers paragraphs
  doc: interpret-trailers: commit to “trailer block” term
  doc: interpret-trailers: join new-trailers again
  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