docs: fix string suggesting HTTP/2 is not the default

Commit 25fd105 made HTTP2 the default, and further down in the
man page that new default is mentioned, but the section at the top
contradicted it until now.

Also remove claim that setting the HTTP version is not sensible.

Closes #4075

docs/libcurl/opts/CURLOPT_HTTP_VERSION.3

@@ -29,9 +29,7 @@ CURLOPT_HTTP_VERSION \- specify HTTP protocol version to use
 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTP_VERSION, long version);
 .SH DESCRIPTION
 Pass \fIversion\fP a long, set to one of the values described below. They ask
-libcurl to use the specific HTTP versions. This is not sensible to do unless
-you have a good reason. You have to set this option if you want to use
-libcurl's HTTP/2 support.
+libcurl to use the specific HTTP versions.
 
 Note that the HTTP version is just a request. libcurl will still prioritize to
 re-use an existing connection so it might then re-use a connection using a