Add note that PMIX_PARENT_ID is only provided for non-tool clients.

I have specifically written this in such a way that ignores the
case where a process is registered as a client (e.g. created through PMIX_Spawn),
but then chooses to call PMIx_tool_init() instead of PMIx_Init().
I did not say, "PMIX_PARENT_ID is only set for pmix clients and not tools".
I also did not preclude having some launcher that is part of the system
(pmixlaunch, mpirun, jsrun, etc) that spawns processes through some means other
than PMIX_Spawn and still sets PMIX_PARENT_ID.  I only say that it must be set
if you were created by PMIX_Spawn.

Signed-off-by:  dsolt@us.ibm.com

Chap_API_Reserved_Keys.tex

@@ -370,6 +370,8 @@ \subsection{Process realm attributes}
 Process identifier of the parent process of the specified process - typically
 used to identify the application process that caused the job containing the
 specified process to be spawned (e.g., the process that called \refapi{PMIx_Spawn}).
+This attribute is only provided for a process if it was created by a call to
+\refapi{PMIx_Spawn} or \refapi{PMIx_Spawn_nb}.
 }
 %
 \declareAttribute{PMIX_EXIT_CODE}{"pmix.exit.code"}{int}{