HTTP2.md: add CURL_HTTP_VERSION_2TLS and updated alt-svc link

docs/HTTP2.md

@@ -37,6 +37,9 @@ If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will use
 ALPN (or NPN) to negotiate which protocol to continue with. Possibly introduce
 an option that will cause libcurl to fail if not possible to use HTTP/2.
 
+`CURL_HTTP_VERSION_2TLS` was added in 7.47.0 as a way to ask libcurl to prefer
+HTTP/2 for HTTPS but stick to 1.1 by default for plain old HTTP connections.
+
 ALPN is the TLS extension that HTTP/2 is expected to use. The NPN extension is
 for a similar purpose, was made prior to ALPN and is used for SPDY so early
 HTTP/2 servers are implemented using NPN before ALPN support is widespread.
@@ -99,7 +102,7 @@ same origin server that you get the response from. A browser or long-living
 client can use that hint to create a new connection asynchronously.  For
 libcurl, we may introduce a way to bring such clues to the applicaton and/or
 let a subsequent request use the alternate route
-automatically. [Spec](https://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-05)
+automatically. [Spec](https://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-14)
 
 TODO
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