rfc14: reference RFC 31 and add example constraints

Problem: RFC 14 does not list RFC 31 in Related Standards, and is also
missing an example jobspec utilizing constraints with properties (the
main use case for job constraints at this point)

Add Use Case 2.9 to show an example usage of property-based job constraints.

Reference RFC 31 in related standards.

Author: grondo

data/spec_14/use_case_2.9.yaml

@@ -0,0 +1,20 @@
+version: 999
+resources:
+  - type: slot
+    count: 1
+    label: default
+    with:
+      - type: node
+        count: 1
+tasks:
+  - command: [ "flux", "start" ]
+    slot: default
+    count:
+      per_slot: 1
+attributes:
+  system:
+    duration: 3600.
+    cwd: "/home/flux"
+    constraints:
+      properties:
+        - amd-mi50

spec_14.rst

@@ -37,6 +37,8 @@ Related Standards
 
 -  :doc:`26/Job Dependency Specification <spec_26>`
 
+-  :doc:`31/Job Constraints Specification <spec_31>`
+
 
 Goals
 -----
@@ -654,6 +656,17 @@ Jobspec YAML
    .. literalinclude:: data/spec_14/use_case_2.8.yaml
       :language: yaml
 
+Use Case 2.9
+   Specify constraints based on properties
+
+Specific Example
+   Require that allocated resources have the ``amd-mi50`` property
+
+Jobspec YAML
+   .. literalinclude:: data/spec_14/use_case_2.9.yaml
+      :language: yaml
+
+
 .. [#f1] `YAML Ain’t Markup Language (YAML) Version 1.1 <http://yaml.org/spec/1.1/current.html>`__, O. Ben-Kiki, C. Evans, B. Ingerson, 2004.
 
 .. [#f2] `JSON Schema: A Media Type for Describing JSON Documents <https://json-schema.org/latest/json-schema-core.html>`__; H. Andrews; 2018