merge revision(s) 39677,39695: [Backport #8068]

	* complex.c: described syntax of string form.

	* rational.c: ditto.

	* complex.c: edited rdoc.

	* rational.c: ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39918 b2dd03c8-39d4-4d8f-98ff-823fe69b080e