rfc42: mention shell-resident exec server

Problem: the job shell's rexec capabilty is not mentioned.

Add it and a use case.

Author: garlick

spec_42.rst

@@ -34,8 +34,7 @@ Related Standards
 Background
 **********
 
-The subprocess server protocol is implemented in two distinct Flux
-components:
+The subprocess server protocol is implemented in three Flux components:
 
 .. list-table::
 
@@ -51,6 +50,10 @@ components:
     - :program:`sdexec`
     - If systemd support is configured
 
+  * - Flux shell
+    - :program:`UID-shell-JOBID.rexec`
+    - Only available to the job owner.
+
 The primary use cases are:
 
 #. The job execution service runs job shells on job nodes.
@@ -59,6 +62,8 @@ The primary use cases are:
 
 #. The instance owner runs arbitrary processes with :program:`flux exec`.
 
+#. Tool launch such as parallel debugger daemons using :program:`flux exec`.
+
 In a multi-user Flux instance where a user transition is necessary in order
 for the instance owner to run commands with the credentials of a guest user,
 the subprocess server delegates this to the IMP.  On its own, the subprocess