threading: clarify openssl default vs mbedtls
diff --git a/docs/threading.md b/docs/threading.md
index 0c31cf6..ddfaf7a 100644
--- a/docs/threading.md
+++ b/docs/threading.md
@@ -58,10 +58,11 @@ general case still affects you if you use ssh.
General Case
------------
-If there are no applicable TLS implementations (currently only
-SecureTransport and mbedTLS), libgit2 defaults to OpenSSL in order to use HTTPS as a transport.
-OpenSSL is thread-safe starting at version 1.1.0. If your copy of libgit2 is
-linked against that version, you do not need to take any further steps.
+libgit2 will default to OpenSSL for HTTPS transport (except on Windows and
+macOS, as mentioned above). On any system, mbedTLS _may_ be optionally
+enabled as the security provider. OpenSSL is thread-safe starting at
+version 1.1.0. If your copy of libgit2 is linked against that version,
+you do not need to take any further steps.
Older versions of OpenSSL are made to be thread-implementation agnostic, and the
users of the library must set which locking function it should use. libgit2