Merge branch 'la/trailer-test-and-doc-updates'

Merge branch 'la/trailer-test-and-doc-updates'

Test coverage for trailers has been improved.

* la/trailer-test-and-doc-updates:
  trailer doc: <token> is a <key> or <keyAlias>, not both
  trailer doc: separator within key suppresses default separator
  trailer doc: emphasize the effect of configuration variables
  trailer --unfold help: prefer "reformat" over "join"
  trailer --parse docs: add explanation for its usefulness
  trailer --only-input: prefer "configuration variables" over "rules"
  trailer --parse help: expose aliased options
  trailer --no-divider help: describe usual "---" meaning
  trailer: trailer location is a place, not an action
  trailer doc: narrow down scope of --where and related flags
  trailer: add tests to check defaulting behavior with --no-* flags
  trailer test description: this tests --where=after, not --where=before
  trailer tests: make test cases self-contained