extension.rdoc: add document title
* doc/extension.rdoc, doc/extension.ja.rdoc: [DOC]
add title and adapt subheading levels.
* doc/extension.rdoc: [DOC] fix subheading level of section
about "Ruby Constants That Can Be Accessed From C".
* doc/extension.ja.rdoc: [DOC] add missing subheading.
[ruby-core:79590] [Bug #13229]
Author: Marcus Stollsteimer <sto.mar@web.de>
merge revision(s) 57665: [Backport #13229]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57910 b2dd03c8-39d4-4d8f-98ff-823fe69b080e