Must not be a bad idea to improve documents. [ci skip]

rb_cFiber is not visible from extension libraries, hence intentionally
not documented. [ci skip]

This enables me to write `@shyouhei` in C comments without complained by
doxygen that @shyouhei is an unknown special command. [ci skip]

If the use of the ruby namespace isn't prepended by `::` to make it
explicit that the global ruby namespace is referenced here, it can
clash with other non-global namespace named ruby, e.g.

```
  // declaration
  namespace myproject {
  namespace ruby {
    // my ruby classes
  }
  }

  // implementation
  using namespace myproject;

  [...]

  rb_define_method(...);

  [...]
```

leads to the following error:

```
../ruby/choice.cc: In function 'void {anonymous}::do_register_choice()':
../ruby/choice.cc:342:9: error: reference to 'ruby' is ambiguous
  342 |         rb_define_method(c_choices, "each", RUBY_FUNC_CAST(&choices_each), 0);
      |         ^~~~~~~~~~~~~~~~
In file included from ../ruby/choice.cc:20:
../ruby/paludis_ruby.hh:53:15: note: candidates are: 'namespace paludis::ruby { }'
   53 |     namespace ruby
      |               ^~~~
In file included from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby/backward/2/stdalign.h:23,
                 from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby/defines.h:77,
                 from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby/ruby.h:23,
                 from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby.h:38,
                 from ../ruby/paludis_ruby.hh:44,
                 from ../ruby/choice.cc:20:
/usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby/internal/stdalign.h:92:11: note:                 'namespace ruby { }'
   92 | namespace ruby {
      |           ^~~~
In file included from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby/internal/anyargs.h:83,
                 from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby/ruby.h:24,
                 from /usr/x86_64-pc-linux-gnu/include/ruby-3.0/ruby.h:38,
                 from ../ruby/paludis_ruby.hh:44,
                 from ../ruby/choice.cc:20:
../ruby/choice.cc:342:9: error: '::define' has not been declared
  342 |         rb_define_method(c_choices, "each", RUBY_FUNC_CAST(&choices_each), 0);
      |         ^~~~~~~~~~~~~~~~
../ruby/choice.cc:344:9: error: reference to 'ruby' is ambiguous
  344 |         rb_define_method(c_choices, "find_by_name_with_prefix", RUBY_FUNC_CAST(&choices_find_by_name_with_prefix), 1);
      |         ^~~~~~~~~~~~~~~~
```

To fix build failures.

This shall fix compile errors.