[DOC] Fix RDoc markup in doc/extension*.rdoc
This patch fixes broken RDoc in extension.rdoc and extension.ja.rdoc.
E.g. fixing labelled lists, comments (# -> -- / ++),
escaping (ary[offset] would else be a link, #to_str would not render correctly,
(c) would render as copyright sign), and some syncing of en/ja.
I'm not sure what happens with '*' at the beginning of a line
(this might be an RDoc bug): it seems to trigger a code block,
so I escaped it or used
<b></b> or used '-' for an unordered list.