As @peterzhu2118 and @duerst have pointed out, putting string method's RDoc into doc/ (which allows non-ASCII in examples) makes the "click to toggle source" feature not work for that method.
This PR moves the primary method doc back into string.c, then includes RDoc from doc/string/*.rdoc, and also removes doc/string.rdoc.
The affected methods are:
::new
#bytes
#each_byte
#each_line
#split
The call-seq is in string.c because it works there; it did not work when the call-seq is in doc/string/*.rdoc.
This PR also updates the relevant guidance in doc/documentation_guide.rdoc.
