doc: update documentation of -q, --queue option where needed

Problem: Multiple queues may now be specified in the `-q, --queue`
option of `flux-jobs(1)`, `flux-pgrep(1)`, and `flux-pkill(1)`, but
this is not documented.

Update the documentation of these commands.

Author: grondo

doc/man1/flux-jobs.rst

@@ -46,9 +46,10 @@ OPTIONS
 
    List jobs with a specific job name.
 
-.. option:: -q, --queue=[QUEUE]
+.. option:: -q, --queue=QUEUE[,...]
 
-   List jobs in a specific queue.
+   List jobs in a specific queue or queues. Multiple queues may be separated
+   by a comma or by using the :option:`-q, --queue` option multiple times.
 
 .. option:: -c, --count=N
 

doc/man1/flux-pgrep.rst

@@ -56,9 +56,11 @@ OPTIONS
    results can be listed separated by comma. See the JOB STATUS section
    of the :man1:`flux-jobs` manual for more detail.
 
-.. option:: -q, --queue=QUEUE
+.. option:: -q, --queue=QUEUE[,...]
 
-   Only include jobs in the named queue *QUEUE*.
+   Only include jobs in the named queue *QUEUE*. Multiple queues may be
+   specified as a comma-separated list, or by using the :option:`--queue`
+   option multiple times.
 
 .. option:: -c, --count=N