Add information about PrologFlags to slurm.conf man page

doc/man/man5/slurm.conf.5

@@ -3743,12 +3743,13 @@ NOTE:  Standard output and error messages are normally not preserved.
 Explicitly write output and error messages to an appropriate location
 if you wish to preserve that information.
 
-NOTE:  The Prolog script is ONLY run on any individual
+NOTE:  By default the Prolog script is ONLY run on any individual
 node when it first sees a job step from a new allocation; it does not
 run the Prolog immediately when an allocation is granted.  If no job steps
 from an allocation are run on a node, it will never run the Prolog for that
-allocation.  The Epilog, on the other hand, always runs on every node of an
-allocation when the allocation is released.
+allocation.  This Prolog behaviour can be changed by the
+\fBPrologFlags\fR parameter.  The Epilog, on the other hand, always
+runs on every node of an allocation when the allocation is released.
 
 If the Epilog fails (returns a non\-zero exit code), this will result in the
 node being set to a DRAIN state.