doc: clarify use of RegisterDynamicBackgroundWorker

Document likely use of RegisterDynamicBackgroundWorker by another
background worker.

Reported-by: Chapman Flack

Discussion: https://postgr.es/m/CAB7nPqTdi=J9HH8PPPiEOohebdd+xkgbbhdY7=VbGnZ3CkZXxA@mail.gmail.com

Author: Chapman Flack

doc/src/sgml/bgworker.sgml

@@ -41,7 +41,7 @@
   *worker, BackgroundWorkerHandle **handle</type>)</function>.  Unlike
   <function>RegisterBackgroundWorker</function>, which can only be called from within
   the postmaster, <function>RegisterDynamicBackgroundWorker</function> must be
-  called from a regular backend.
+  called from a regular backend, possibly another background worker.
  </para>
 
  <para>