Merge branch 'jc/fetch-pull-refmap'

* jc/fetch-pull-refmap:
  docs: Explain the purpose of fetch's and pull's <refspec> parameter.
  fetch: allow explicit --refmap to override configuration
  fetch doc: add a section on configured remote-tracking branches
  fetch doc: remove "short-cut" section
  fetch doc: update refspec format description
  fetch doc: on pulling multiple refspecs
  fetch doc: remove notes on outdated "mixed layout"
  fetch doc: update note on '+' in front of the refspec
  fetch doc: move FETCH_HEAD material lower and add an example
  fetch doc: update introductory part for clarity