Merge branch 'ja/doc-synopsis-style'

Doc mark-up fixes.

* ja/doc-synopsis-style:
  doc: convert git-switch manpage to new synopsis style
  doc: convert git-mergetool options to new synopsis style
  doc: convert git-mergetool manpage to new synopsis style
  doc: switch merge config description to new synopsis format
  doc: convert merge strategies to synopsis format
  doc: merge-options.adoc remove a misleading double negation
  doc: convert merge options to new synopsis format
  doc: convert git-merge manpage to new style
  doc: convert git-checkout manpage to new style