flux-exec(1): add CAVEATS

Problem: flux-exec(1) does not note the rank 0 restrictions, nor
does it make it clear that the instance owner has no special powers
when it comes to the --jobid option.

Add a CAVEATS section that explains these constraints.

Author: garlick

doc/man1/flux-exec.rst

@@ -100,6 +100,14 @@ OPTIONS
    signal remote commands instead of the normal builtin subprocess signaling
    mechanism.
 
+CAVEATS
+=======
+
+In a multi-user flux instance, access to the rank 0 broker execution
+service is restricted to requests that originate from the local broker.
+Therefore, :program:`flux exec` (without :option:`--jobid`) must be run
+from the rank 0 broker if rank 0 is included in the target *IDSET*.
+
 EXIT STATUS
 ===========