[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>

This makes `RBobject` `4B` larger on 32 bit systems
but simplifies the implementation a lot.

[Feature #21353]

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

[Bug #20585]

This was changed in 36a06efdd9f0604093dccbaf96d4e2cb17874dc8 because
`String.new(1024)` would end up allocating `1025` bytes, but the problem
with this change is that the caller may be trying to right size a String.

So instead, we should just better document the behavior of `capacity:`.

[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.

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>

The documentation for `rb_enc_interned_str_cstr` notes that `enc` can be
a null pointer, but this currently causes a segmentation fault when
trying to autoload the encoding. This commit fixes the issue by checking
for NULL before calling `rb_enc_autoload`.

  Encoding::Windows_31J is already loaded in mswin platform

If the required capacity would fit in an embded string,
returns one.

This can reduce malloc churn for code that use string buffers.