MEDIUM: config: report that "nbproc" is deprecated

As previously discussed, nbproc usage is bad, deprecated, and scheduled
for removal in 2.5.

If "nbproc" is found with more than one process while nbthread is not
set, a warning will be emitted encouraging to remove it or to migrate
to nbthread instead. This makes sure the user has an opportunity to
both see the message and silence it.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index b0edb96..bd9781c 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1176,14 +1176,15 @@
   SIGTERM. This option helps to keep under control the number of workers.
   See also "show proc" in the Management Guide.
 
-nbproc <number>
+nbproc <number>  (deprecated)
   Creates <number> processes when going daemon. This requires the "daemon"
   mode. By default, only one process is created, which is the recommended mode
   of operation. For systems limited to small sets of file descriptors per
   process, it may be needed to fork multiple daemons. When set to a value
   larger than 1, threads are automatically disabled. USING MULTIPLE PROCESSES
-  IS HARDER TO DEBUG AND IS REALLY DISCOURAGED. See also "daemon" and
-  "nbthread".
+  IS HARDER TO DEBUG AND IS REALLY DISCOURAGED. This directive is deprecated
+  and scheduled for removal in 2.5. Please use "nbthread" instead. See also
+  "daemon" and "nbthread".
 
 nbthread <number>
   This setting is only available when support for threads was built in. It