CURLOPT_ALTSVC_CTRL.3: mention conn reuse is preferred

Ref: https://github.com/curl/curl/discussions/7954

docs/libcurl/opts/CURLOPT_ALTSVC_CTRL.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -38,11 +38,15 @@ CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC_CTRL, long bitmask);
 Populate the long \fIbitmask\fP with the correct set of features to instruct
 libcurl how to handle Alt-Svc for the transfers using this handle.
 
-libcurl will only accept Alt-Svc headers over a secure transport, meaning
+libcurl only accepts Alt-Svc headers over a secure transport, meaning
 HTTPS. It will also only complete a request to an alternative origin if that
 origin is properly hosted over HTTPS. These requirements are there to make
 sure both the source and the destination are legitimate.
 
+Alternative services are only used when setting up new connections. If there
+exists an existing connection to the host in the connection pool, then that
+will be preferred.
+
 Setting any bit will enable the alt-svc engine.
 .IP "CURLALTSVC_READONLYFILE"
 Do not write the alt-svc cache back to the file specified with