setup: limits: note about reaching limits

It is not the first time we have people not understanding why only <X
additional subflows have been established, while the limit is X.

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

setup.md

@@ -185,6 +185,14 @@ ip mptcp limits set [ subflows NR ] [ add_addr_accepted NR ]
   notification from the other peer -- that will result in the creation of
   subflows, typically only for the client side (default is 0).
 
+{: .note}
+It is possible to reach the limits with fewer established subflows than
+expected, e.g. when new subflow requests cannot reach the other peer. In case of
+problem, please increase the limits, use `ss -Mai` to check the counters, and
+set the adapted routing or firewall rules to avoid using certain paths between
+specific IP addresses, e.g. in a lab setup with dedicated links, don't use
+default but specific routes.
+
 ### Manual routing configuration
 
 <details markdown="block">