doc: update description of --tasks-per-core in flux-mini(1)

Problem: A lack of exposition on the --tasks-per-core option of
flux-mini may lead users astray in thinking this option is more useful
than it really is.

Move the --tasks-per-core option to the end of the Per-resource
options list, thereby showing it as the lesser sibling of this option
group. Expand the option description to include the many caveats,
warnings, and a final note that most likely this is not the option
you want.

Author: grondo

doc/man1/flux-mini.rst

@@ -116,12 +116,17 @@ listed above:
 **--tasks-per-node=N**
    Set the number of tasks per node to run.
 
-**--tasks-per-core=N**
-   Force a number of tasks per core.
-
 **--gpus-per-node=N**
    With -N, --nodes, request a specific number of GPUs per node.
 
+**--tasks-per-core=N**
+   Force a number of tasks per core. Note that this will run *N* tasks per
+   *allocated* core. If nodes are exclusively scheduled by configuration or
+   use of the ``--exclusive`` flag, then this option could result in many
+   more tasks than expected. The default for this option is effectively 1,
+   so it is useful only for oversubscribing tasks to cores for testing
+   purposes. You probably don't want to use this option.
+
 Batch job options
 -----------------