Merge branch 'ja/doc-placeholders-markup-rules' into HEAD

The way placeholders are to be marked-up in documentation have been
specified; use "_<placeholder>_" to typeset the word inside a pair
of <angle-brakets> emphasized.

* ja/doc-placeholders-markup-rules:
  doc: clarify the format of placeholders

diff --cc Documentation/CodingGuidelines
Simple merge