MINOR: doc: specify ulimit-n usage for dynamic servers

Complete the documentation of the dynamic servers to warn about a
possible fd resource exhaustion when using a large number of them.
diff --git a/doc/management.txt b/doc/management.txt
index dbc3229..0fb838a 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1485,6 +1485,10 @@
   be first deactivated via "disable agent" to be able to put the server in the
   required maintenance mode before removal.
 
+  It may be possible to reach the fd limit when using a large number of dynamic
+  servers. Please refer to the "u-limit" global keyword documentation in this
+  case.
+
   Here is the list of the currently supported keywords :
 
   - agent-addr