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.

Refine the uses of word "Data", which were often ambiguous.  Also,
that word now refers the new class unrelated to `T_DATA`.