doc: document job shell hostfile taskmap scheme

Problem: The hostfile taskmap scheme is not documented.

Mention this scheme in flux-shell(1) along with the other builtin
taskmap schemes, and document it in the cli submission commands.

Author: grondo

doc/man1/common/job-other-run.rst

@@ -17,6 +17,14 @@
     the job shell, so this option is not useful unless the total number
     of nodes and tasks per node are known at job submission time.
 
+   hostfile:FILE
+    Assign tasks in order to hosts as they appear in FILE. FILE should
+    have one or more lines each of which contains a host name or RFC
+    29 Hostlist string. Each host assigned to the job must appear in
+    the hostfile and be assigned the same number of tasks as the default
+    taskmap from the shell.  If there are less hosts in the hostfile than
+    tasks in the job, then the list of hosts will be reused.
+
    However, shell plugins may provide other task mapping schemes, so
    check the current job shell configuration for a full list of supported
    taskmap schemes.

doc/man1/flux-shell.rst

@@ -126,13 +126,13 @@ topics:
 **taskmap.SCHEME**
   Called when a taskmap scheme *SCHEME* is requested via the taskmap
   shell option or corresponding :option:`flux submit --taskmap` option.
-  Plugins that want to offer a different taskmap scheme than the defaults of
-  ``block``, ``cyclic``, and ``manual`` can register a ``taskmap.*`` plugin
-  callback and then users can request this mapping with the appropriate
-  :option:`flux submit --taskmap=name`` option.  The default block taskmap is
-  passed to the plugin as "taskmap" in the plugin input arguments, and the
-  plugin should return the new taskmap as a string in the output args.  This
-  callback is called before ``shell.init``.
+  Plugins that want to offer a different taskmap scheme than the defaults
+  of ``block``, ``cyclic``, ``hostfile``, and ``manual`` can register a
+  ``taskmap.*`` plugin callback and then users can request this mapping
+  with the appropriate :option:`flux submit --taskmap=name`` option.
+  The default block taskmap is passed to the plugin as "taskmap" in the
+  plugin input arguments, and the plugin should return the new taskmap as a
+  string in the output args.  This callback is called before ``shell.init``.
 
 **shell.connect**
   Called just after the shell connects to the local Flux broker. (Only