Convert a TODO(3.0) in OPENSSL_thread_stop_ex to a comment
Convert a TODO(3.0) in OPENSSL_thread_stop_ex to a comment

The TODO is describing something that would be nice to fix. In fact the
problem exists even in 1.1.1. It would be nice to fix it, but it does
not need to be done in the 3.0 timeframe.

Fixes #14376

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14533)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl