doc: clarify stats output in flux-jobs(1)

Problem: The flux-jobs(1) --stats and --stats-only output count
all job failures ("failed", "timeout", "canceled") as a single
statistic and output that as "failed".  But this may not be obvious
because job results are listed as completed, failed, timeout, and
canceled.

Add a clarification under the --stats and --stats-only descriptions.

Author: chu11

doc/man1/flux-jobs.rst

@@ -104,6 +104,9 @@ OPTIONS
    will display a summary of statistics along with the top 25
    running jobs, updated every 2 seconds.
 
+   Note that all job failures, including canceled and timeout jobs,
+   are collectively counted as "failed" in ``--stats``.
+
 **--stats-only**
    Output a summary of job statistics and exit.  By default shows
    global statistics.  If ``--queue`` is specified, shows statistics
@@ -116,6 +119,9 @@ OPTIONS
    All options other than ``--queue`` are ignored when
    ``--stats-only`` is used.
 
+   Note that all job failures, including canceled and timeout jobs,
+   are collectively counted as "failed" in ``--stats-only``.
+
 **-R, --recursive**
    List jobs recursively. Each child job which is also an instance of
    Flux is prefixed by its jobid "path" followed by the list of jobs,