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](psf/requests#6667) where the matter was clarified by @tiran in [this comment](psf/requests#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.

Author: mm-matthias

Doc/library/ssl.rst

@@ -1472,6 +1472,19 @@ to speed up repeated connections from the same clients.
       :data:`PROTOCOL_TLS`, :data:`PROTOCOL_TLS_CLIENT`, and
       :data:`PROTOCOL_TLS_SERVER` use TLS 1.2 as minimum TLS version.
 
+   .. note::
+
+      :class:`SSLContext` only supports limited mutation once it has been used
+      by a connection. Adding new certificates to the internal trust store is
+      allowed, but changing ciphers, verification settings, or mTLS
+      certificates may result in surprising behavior.
+
+   .. note::
+
+      :class:`SSLContext` is designed to be shared and used by multiple
+      connections.
+      Thus, it is thread-safe as long as it is not reconfigured after being
+      used by a connection.
 
 :class:`SSLContext` objects have the following methods and attributes: