Due to the bug ruby/rdoc#1067, `rb_file_const` needs `Document-const`
directives.

* Reword Range#overlap? docs last paragraph.

* Docs: add explanation about Queue#freeze

* Docs: Add :rescue event docs for TracePoint

* Docs: Enhance Module#set_temporary_name documentation

* Docs: Slightly expand Process::Status deprecations

* Fix MatchData#named_captures rendering glitch

* Improve Dir.fchdir examples

* Adjust Refinement#target docs

If no block is given, return 0 instead of nil for consistency
with Dir.chdir and Dir.fchdir.

Mostly just fixing RDoc's incorrect treatment of `+`

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.

This patch introduce M:N thread scheduler for Ractor system.

In general, M:N thread scheduler employs N native threads (OS threads)
to manage M user-level threads (Ruby threads in this case).
On the Ruby interpreter, 1 native thread is provided for 1 Ractor
and all Ruby threads are managed by the native thread.

From Ruby 1.9, the interpreter uses 1:1 thread scheduler which means
1 Ruby thread has 1 native thread. M:N scheduler change this strategy.

Because of compatibility issue (and stableness issue of the implementation)
main Ractor doesn't use M:N scheduler on default. On the other words,
threads on the main Ractor will be managed with 1:1 thread scheduler.

There are additional settings by environment variables:

`RUBY_MN_THREADS=1` enables M:N thread scheduler on the main ractor.
Note that non-main ractors use the M:N scheduler without this
configuration. With this configuration, single ractor applications
run threads on M:1 thread scheduler (green threads, user-level threads).

`RUBY_MAX_CPU=n` specifies maximum number of native threads for
M:N scheduler (default: 8).

This patch will be reverted soon if non-easy issues are found.

[Bug #19842]