docs: schannel and darwinssl documentation improvements

Schannel and darwinssl use the certificates built into the
OS to do vert verification instead of bundles. darwinssl
is thread-safe. Corrected typos in the NSS docs.

docs/libcurl/libcurl-tutorial.3

@@ -289,6 +289,10 @@ axTLS
 
  Required actions unknown.
 
+Secure Transport
+
+ The engine is fully thread-safe, and no additional steps are required.
+
 When using multiple threads you should set the CURLOPT_NOSIGNAL option to 1
 for all handles. Everything will or might work fine except that timeouts are
 not honored during the DNS lookup - which you can work around by building