rfc20: expand description of scheduler specific properties

Problem: The purpose of the scheduler-specific `@` character in
property strings is vague in RFC 20, and could cause confusion.

Expand the description of the purpose and possible use case for `@`
in execution target properties. Specify that the literal property
string shall still apply to the execution target, but the scheduler
implementation may use all or part of the prefix and suffix before the
`@` for scheduler specific optimizations.

Author: grondo

spec_20.rst

@@ -175,11 +175,15 @@ The ``execution`` key MAY also contain any of the following optional keys:
       ! & ' " ^ ` | ( )
 
    Additionally, the ``@`` character is reserved for scheduler specific
-   property use. Any suffix that appears after the ``@`` character in a
-   property name SHALL be a scheduler-specific string. For example,
-   ``amd-mi50@gpu``, ``amd-mi50`` SHALL be the property string, but a
-   scheduler MAY use the ``gpu`` suffix to perform scheduling optimization
-   for gpus of the corresponding ranks.
+   property use. In this case, the literal property SHALL still apply
+   to the defined execution target ranks, but the scheduler MAY use the
+   suffix after ``@`` to apply the property to children resources of the
+   execution target or for another scheduler specific purpose. For example,
+   the property ``amd-mi50@gpu`` SHALL apply to the defined execution
+   target ranks, but a scheduler MAY use the ``gpu`` suffix to perform
+   scheduling optimization for gpus of the corresponding ranks. This MAY
+   result in both ``amd-mi50@gpu`` and ``amd-mi50`` being valid properties
+   for resources in the instance.
 
 **starttime**
    The value of the ``starttime`` key, if present, SHALL