DOC: explain HTTP2 timeout behavior

Clarifies that in HTTP2 we don't consider "timeout http-keep-alive", but
"timeout client" instead.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e756d41..c1b6d74 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -9855,6 +9855,8 @@
   during startup because it may results in accumulation of expired sessions in
   the system if the system's timeouts are not configured either.
 
+  This also applies to HTTP2 connections, which will be closed with GOAWAY.
+
   This parameter replaces the old, deprecated "clitimeout". It is recommended
   to use it to write new configurations. The form "timeout clitimeout" is
   provided only by backwards compatibility but its use is strongly discouraged.
@@ -9960,6 +9962,10 @@
   set in the frontend to take effect, unless the frontend is in TCP mode, in
   which case the HTTP backend's timeout will be used.
 
+  When using HTTP2 "timeout client" is applied instead. This is so we can keep
+  using short keep-alive timeouts in HTTP/1.1 while using longer ones in HTTP2
+  (where we only have one connection per client and a connection setup).
+
   See also : "timeout http-request", "timeout client".