doc/go1.7.html: document that http.Server now enforces request versions

Document that the http.Server is now stricter about rejecting
requests with invalid HTTP versions, and also that it rejects plaintext
HTTP/2 requests, except for `PRI * HTTP/2.0` upgrade requests.
The relevant CL is https://golang.org/cl/24505.

Updates #15810.

Change-Id: Ibbace23e001b5e2eee053bd341de50f9b6d3fde8
Reviewed-on: https://go-review.googlesource.com/24731Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.7.html

@@ -896,6 +896,13 @@ The server implementation now correctly sends only one "Transfer-Encoding" heade
 is set explicitly, following <a href="https://tools.ietf.org/html/rfc7230#section-3.3.1">RFC 7230</a>.
 </p>
 
+<p>
+The server implementation is now stricter about rejecting requests with invalid HTTP versions.
+Invalid requests claiming to be HTTP/0.x are now rejected (HTTP/0.9 was never fully supported),
+and plaintext HTTP/2 requests other than the "PRI * HTTP/2.0" upgrade request are now rejected as well.
+The server continues to handle encrypted HTTP/2 requests.
+</p>
+
 <p>
 In the server, a 200 status code is sent back by the timeout handler on an empty
 response body, instead of sending back 0 as the status code.