doc: document sort options in flux-jobs(1)

Problem: The --sort option as well as the sort: prefix for format
strings is not documented in flux-jobs(1).

Document the two methods to establish an output sort order in
flux-jobs(1).

doc/man1/flux-jobs.rst

@@ -101,6 +101,13 @@ OPTIONS
    configuration snippet for an existing named format may be generated with
    :option:`--format=get-config=NAME`.
 
+.. option:: --sort=[-]KEY,..
+
+   Sort jobs based on a list of comma separated keys. If a KEY is preceded
+   by a dash ``-``, then the sort order is reversed. Supported keys match
+   output field names, e.g. ``id``, ``t_run``, etc. This option overrides
+   any ``sort:`` prefix specified in the current format.
+
 .. option:: --json
 
    Emit data for selected jobs in JSON format. The data for multiple
@@ -228,6 +235,12 @@ following is the format used for the default format:
    {status_abbrev:>2.2} {ntasks:>6} {nnodes:>6h} \
    {contextual_time!F:>8h} {contextual_info}
 
+If the format string begins with ``sort:k1[,k2,...]``, then ``k1[,k2,...]``
+will be taken to be a comma-separated list of keys on which to sort
+the displayed output. If a sort key starts with ``-``, then the key
+will be sorted in reverse order. The sort order embedded in the format
+ may be overridden on the command line by the :option:`--sort` option.
+
 If a format field is preceded by the special string ``?:`` this will
 cause the field to be removed entirely from output if the result would
 be an empty string or zero value for all jobs in the listing. E.g.::