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.