`:nodoc:` seems not working for inner classes.

Rarely, there are cases where a write(2) call from a child thread
to notify the main thread of the completion of name resolution fails.
If this happens while the main thread is waiting in `rb_thread_fd_select`,
rb_thread_fd_select may not notice that the name resolution has completed and end up hanging.

This issue becomes a problem when there are no sockets currently being connected,
no addresses ready for immediate connection attempts,
and name resolution has already completed for one address family
while the main thread is waiting for the name resolution of the other address family.
(If name resolution is not completed for either address family,
the chances of write(2) failing in both child threads are likely low.)

To avoid this issue, a timeout is introduced to rb_thread_fd_select under the above conditions.
This way, even if the issue occurs,
the completion of name resolution should still be detected
in the subsequent `if (!resolution_store.is_all_finished) ...` block.

Also, a topic about Socket::ResolutionError is added to NEWS

to prevent the following scenario:

1. `delete_unique_str()` can be called while GC (sweeping)
2. it calls `st_insert()` to decrement the counter
3. `st_insert()` can try to extend the table even if the key exists
4. `xmalloc` while GC and cause BUG

https://github.com/ruby/openssl/commit/e5153dbbb4

OpenSSL::Digest#finish overrides Digest::Instance#finish and is called
from the Digest::Class framework in the digest library. This method is
not supposed to take any arguments, as suggested by the RDoc comment for
Digest::Instance#finish.

It is a private method and not exposed to users. Let's remove it.

This optional parameter exists since r15602 in Ruby trunk, the commit
which converted OpenSSL::Digest to a subclass of Digest::Class.

https://github.com/ruby/openssl/commit/dcb2a4f30b

Likewise, OpenSSL::Digest#finish needs to make the output buffer
independent before writing to it.

https://github.com/ruby/openssl/commit/9cc8a83466

OpenSSL::Cipher#update accepts a String as the second argument to be
used as the output buffer. The buffer must be directly writable, in
other words, it must not be frozen and not a shared string.

rb_str_resize() does not make the String independent if the String
already has the intended length. Use the rb_str_modify() family instead
to check it.

Fixes: https://bugs.ruby-lang.org/issues/20937

https://github.com/ruby/openssl/commit/1de3b80a46