doc: document job shell output.limit option

Problem: The job shell output.limit option is not documented.

Document this option in flux-shell(1) as well the common job shell
options table for the cli submission commands.

doc/man1/common/job-shell-options.rst

@@ -39,3 +39,8 @@
 
    * - :option:`hwloc.xmlfile`
      - Write hwloc XML gathered by job to a file and set ``HWLOC_XMLFILE``
+
+   * - :option:`output.limit`
+     - Set KVS output limit to SIZE bytes, where SIZE may be a floating point
+       value including optional SI units: k, K, M, G. This value is ignored
+       if output is directed to a file with :option:`--output`.

doc/man1/flux-shell.rst

@@ -318,6 +318,14 @@ plugins include:
   this option directly, as it will be set automatically by options
   of higher level commands such as :man1:`flux-submit`.
 
+.. option:: output.limit=SIZE
+
+   Truncate KVS output after SIZE bytes have been written. SIZE may
+   be a floating point value with optional SI units k, K, M, G. A value of
+   0 is considered unlimited. The default KVS output limit is 10M for jobs
+   in a multi-user instance or unlimited for single-user instance jobs.
+   This value is ignored if output is directed to a file.
+
 .. option:: output.{stdout,stderr}.path=PATH
 
   Set job stderr/out file output to PATH.