MINOR: spoe: Add max-waiting-frames directive in spoe-agent configuration

This is the maximum number of frames waiting for an acknowledgement on the same
connection. This value is only used when the pipelinied or asynchronus exchanges
between HAProxy and SPOA are enabled. By default, it is set to 20.
diff --git a/doc/SPOE.txt b/doc/SPOE.txt
index 7ce2bef..cde3b1d 100644
--- a/doc/SPOE.txt
+++ b/doc/SPOE.txt
@@ -167,6 +167,7 @@
     - maxconnrate
     - maxerrrate
     - max-frame-size
+    - max-waiting-frames
     - messages
     - [no] option async
     - [no] option pipelining
@@ -209,6 +210,10 @@
   It must be in the range [256, tune.bufsize-4] (4 bytes are reserved for the
   frame length). By default, it is set to (tune.bufsize-4).
 
+max-waiting-frames <number>
+  Set the maximum number of frames waiting for an acknowledgement on the same
+  connection. This value is only used when the pipelinied or asynchronus
+  exchanges between HAProxy and SPOA are enabled. By default, it is set to 20.
 
 messages <msg-name> ...
   Declare the list of SPOE messages that an agent will handle.