[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>

rb_str_hash_cmp returns 0 if the two strings are identical and 1 if they
are different.

It is not "in bytes" for wide char literal.

[Feature #20205]

Now that chilled strings no longer appear as frozen, there is no
need to offer an API to check for chilled strings.

We however need to change `rb_check_frozen_internal` to no
longer be a macro, as it needs to check for chilled strings.

Signed-off-by: crazeteam <lilujing@outlook.com>

Some extensions (like stringio) may need to differentiate between
chilled strings and frozen strings.

They can now use rb_str_chilled_p but must check for its presence since
the function will be removed when chilled strings are removed.

[Bug #20389]

[Feature #20205]

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

Signed-off-by: cui fliter <imcusg@gmail.com>

Already these had been announced to be removed in 3.2.