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.

doc/configuration.txt

@@ -2006,6 +2006,10 @@ ulimit-n <number>
   default, it is automatically computed, so it is recommended not to use this
   option.
 
+  Note that the dynamic servers are not taken into account in this automatic
+  resource calculation. If using a large number of them, it may be needed to
+  manually specify this value.
+
 unix-bind [ prefix <prefix> ] [ mode <mode> ] [ user <user> ] [ uid <uid> ]
           [ group <group> ] [ gid <gid> ]
 

doc/management.txt

@@ -1485,6 +1485,10 @@ add server <backend>/<server> [args]*
   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