bind_textdomain_codeset() return value updates

bind_textdomain_codeset() is documented to return a string on success, and false on failure. But there is one success case that also returns false. If you query the codeset for a domain that has not been explicitly set (yet), you will also get false:

php > var_dump(bind_textdomain_codeset("foo", NULL));
bool(false)

This is because the C function returns NULL to indicate that the locale's codeset will be used. Quoting from https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/functions/bind_textdomain_codeset.html

If codeset is a null pointer and domainname is a non-empty string, bind_textdomain_codeset() shall return the current codeset for the named domain, or a null pointer if a codeset has not yet been set.

I bring this up because musl only supports UTF-8, and always returns NULL from its bind_textdomain_codeset(). Its PHP counterpart therefore always appears to fail, when it is working as... well, not quite expected, but as documented.

To summarize:

• false will also be returned if no codeset has been explicitly set for the domain
• this function always returns false under musl libc where the only supported codeset is UTF-8

Edit: as of musl-1.2.6, a NULL is returned only when the requested codeset is not "UTF-8" (https://git.musl-libc.org/cgit/musl/commit/src/locale/bind_textdomain_codeset.c?id=cabbd8697d39c09ca37fb3d63b86b42526b81fda)

Start at the PHP documentation entry for bind_textdomain_codeset() and review its documented return values alongside the POSIX and musl behavior cited in the issue. Update the return-value description to cover the unset-codeset and musl cases, then verify that the documentation accurately distinguishes success from failure.