flux-start(1): document taskmap capability

Problem: flux-start --test-pmi-clique=TASKMAP is undocumented.

Note that capability in the man page.

Author: garlick

doc/man1/flux-start.rst

@@ -207,8 +207,8 @@ OPTIONS
    Set the pmi clique mode, which determines how ``PMI_process_mapping`` is set
    in the PMI server used to bootstrap the brokers.  If ``none``, the mapping
    is not created.  If ``single``, all brokers are placed in one clique. If
-   ``per-broker``, each broker is placed in its own clique.
-   Default: ``single``.
+   ``per-broker``, each broker is placed in its own clique.  Otherwise the
+   option argument is interpreted as an RFC 34 taskmap.  Default: ``single``.
 
 .. option:: -r, --recovery=[TARGET]