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>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57910 b2dd03c8-39d4-4d8f-98ff-823fe69b080e