* [DOC] Cross-links between Japanese and English pages

In rdoc files, all documents belong to the top level of each file, and
`:enddoc:` turns off `#document_self` for the entire document in the
current file.  This disables cross-references to labels in that file.
Instead use `:stopdoc:` so that parsing can re-start by `:startdoc:`
after the local variables.

Instead of `RUBY_REFERENCES_START` and `RUBY_REFERENCES_END`, so that
auto-indent works well.

Also move `struct` so that `typedef`-ed names can be used.

- RDoc is not markdown, use `+` and `_` for code and variables
  respectively, instead of backquotes.

- Remove useless backslashes.