MINOR: h2: expose tune.h2.max-concurrent-streams to limit the number of streams

This will be advertised in the settings frame.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e64cd32..8ac1fb6 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -601,6 +601,7 @@
    - tune.comp.maxlevel
    - tune.h2.header-table-size
    - tune.h2.initial-window-size
+   - tune.h2.max-concurrent-streams
    - tune.http.cookielen
    - tune.http.logurilen
    - tune.http.maxhdr
@@ -1386,6 +1387,15 @@
   faster uploads, or to reduce it to increase fairness when dealing with many
   clients. It doesn't affect resource usage.
 
+tune.h2.max-concurrent-streams <number>
+  Sets the HTTP/2 maximum number of concurrent streams per connection (ie the
+  number of outstanding requests on a single connection). The default value is
+  100. A larger one may slightly improve page load time for complex sites when
+  visited over high latency networks, but increases the amount of resources a
+  single client may allocate. A value of zero disables the limit so a single
+  client may create as many streams as allocatable by haproxy. It is highly
+  recommended not to change this value.
+
 tune.http.cookielen <number>
   Sets the maximum length of captured cookies. This is the maximum value that
   the "capture cookie xxx len yyy" will be allowed to take, and any upper value