gh-118596: Add thread-safety clarifications to the SSLContext documentation (#118597)

gh-118596: Add thread-safety clarifications to the SSLContext documentation (#118597)

Add thread-safety clarifications to the SSLContext documentation. Per the issue:

This issue has also come up [here](https://github.com/psf/requests/pull/6667) where the matter was clarified by @tiran in [this comment](https://github.com/psf/requests/pull/6667):
> `SSLContext` is designed to be shared and used for multiple connections. It is thread safe as long as you don't reconfigure it once it is used by a connection. Adding new certs to the internal trust store is fine, but changing ciphers, verification settings, or mTLS certs can lead to surprising behavior. The problem is unrelated to threads and can even occur in a single-threaded program.