[PATCH] [Bug #21842] Let `rb_interned_str` return US-ASCII if possible

	[PATCH] [DOC] Update docs for rb_interned_str and related functions (#15897)

	Related to [Bug #21842].

	* rb_interned_str: document what decides whether the returned string is
	  in US-ASCII or BINARY encoding.
	* rb_interned_str_cstr: include the same description as rb_interned_str
	  for the encoding. This one was still missing the update for US-ASCII
	  and erroneously said the returned string was alwasy in BINARY encoding
	* rb_str_to_interned_str: document how the encoding of the result is
	  defined.

	Co-authored-by: Herwin <herwinw@users.noreply.github.com>

Allows to remove some duplicated code like szqueue_length, etc.

* `rb_intern_str`: the argument must be `T_STRING`, no conversion.

* `rb_intern_str`, `rb_check_id`, `rb_to_id`, `rb_check_symbol`: raise
  `EncodingError` unless the "name" argument is a valid string in its
  encoding.

Found via `grep` for repeated words.

* set.c: Fix "or or"
* include/ruby/internal/symbol.h: Fix "is is"
* include/ruby/internal/ctype.h: Fix "in in"

This struct is used for addition not only for multiplication, so
remove the word `mul`, and make the member names more descriptive.

This guard was removed in https://github.com/ruby/ruby/pull/13497
on the justification that some GC may need to be notified even for
immediate.

But the two currently available GCs don't, and there are plenty
of assumtions GCs don't everywhere, notably in YJIT and ZJIT.

This optimization is also not so micro (but not huge either).
I routinely see 1-2% wasted there on micro-benchmarks.

So perhaps if in the future we actually need this, it might make
sense to introduce a way for GCs to declare that as an option,
but in the meantime it's extra overhead with little gain.

The changes are to `io.c` and `thread.c`.
I changed the API of 2 exported thread functions from `internal/thread.h` that
didn't look like they had any use in C extensions:

* rb_thread_wait_for_single_fd
* rb_thread_io_wait

I didn't change the following exported internal function because it's
used in C extensions:

* rb_thread_fd_select

I added a comment to note that this function, although internal, is used
in C extensions.

Since support for Windows 9x was dropped over a decade ago.