gtk3-gi: don't mark raw APIs private

Because advanced users can use raw APIs at your own risk.
gtk3-gi: show expected argument and actual argument
gtk3-gi: make brackets needless of array argument

    Gtk::TreeStore.new([String, String, String, TrueClass])
                       ^                                 ^

Maybe we should do it in Ruby/GObjectIntrospection.
Migrate to Markdown from RD
Use GitHub issues for current bug tracker
Try to use definition list markup
Revert "Try to use definition list markup"

This reverts commit 410126cfbc09d0432632d8d5d377f711ad374e8f.

We can't use definition list markup on GitHub. :<
It is better that we use nested list instead of HTML like:

    * `--ruby`
      * ruby directory
    * `--top-srcdir`
      * ...
README.md: drop rcairo(optional) description

Ruby-GNOME2 gems already require rcairo as dependency.
ref: #193