[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>