rfc14: update definition of duration=0

Problem: RFC 14 defines a jobspec duration of 0 as being unset or
unlimited, but does not specify further what that means. This could
allow different scheduler implementations to drift in how they apply
time limits when no duration is specified.

Add wording that explicitly indicates how a scheduler should treat an
unset duration.

Author: grondo

spec_14.rst

@@ -316,11 +316,16 @@ which, if present, must have values. Values MAY have any valid YAML type.
 Some common system attributes are:
 
 **duration**
-   The value of the ``duration`` attribute is a floating-point number greater than
-   or equal to zero representing time span in seconds. A ``duration`` of 0 SHALL be
-   interpreted as "unlimited" or "not set" by the system. The scheduler will make
-   an effort to allocate the requested resources for the number of seconds
-   specified in ``duration``.
+   The value of the ``duration`` attribute is a floating-point number
+   greater than or equal to zero representing time span in seconds.
+   If ``duration`` is greater than zero, then the scheduler SHALL
+   allocate the requested resources for the number of seconds specified
+   in ``duration``. A ``duration`` value of ``0.`` SHALL be interpreted
+   as unlimited or unset, and the scheduler SHALL use a ``duration``
+   that is the minimum of any configured ``duration`` default and the
+   time span to the currently defined instance expiration. If no default
+   ``duration`` is configured, and the current instance has no expiration,
+   then resources shall be allocated without expiration.
 
 **environment**
    The value of the ``environment`` attribute is a dictionary containing the names