merge revision(s) 54110: [Backport #12143]
* doc/extension.ja.rdoc: Fix RDoc markup in doc/extension*.rdoc. [ci skip][Bug #12143][ruby-core:74143] * doc/extension.rdoc: ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@54415 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merge revision(s) 54110: [Backport #12143]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@54415 b2dd03c8-39d4-4d8f-98ff-823fe69b080e