doc: mention the necessity to configure enough file descriptors

doc/configuration.rst

@@ -693,6 +693,17 @@ or I/O response during the operation. It may be caused by Knot design not fittin
 The user should try increasing the number of workers (of the related type) slightly above 100 and if
 the performance gets better, he can decide about further exact setting.
 
+Number of available file descriptors
+------------------------------------
+
+A name server configured for higher number of zones (hundreds and more) needs enough file descriptors
+available for zone transfers and zone file updates, which a default OS setting often doesn't provide.
+It's necessary to check with the OS configuration and documentation and make sure the number of file
+descriptors (sometimes called a number of concurrently open files) effective for the knotd process
+is set high enough. The number of concurrently open incoming TCP connections must be taken into
+account too, in other words, the required setting is affected by the :ref:`server_tcp-max-clients`
+setting.
+
 Sysctl and NIC optimizations
 ----------------------------