Backport of docs: document handling of unset affinity/constraint values (#26354) (#26381)

Affinities and contraints use similar feasibility checking logic to determine if
a given node matches (although affinities don't support all the same
operators). Most operators don't allow `value` to be unset. Update the docs to
reflect this.

Fixes: #24983

Co-authored-by: Tim Gross <[email protected]>

Author: hc-github-team-nomad-core

website/content/docs/job-specification/affinity.mdx

@@ -87,10 +87,11 @@ restricted only to nodes that meet the constraint's criteria.
   For a detailed explanation of these values and their behavior, please see
   the [operator values section](#operator-values).
 
-- `value` `(string: "")` - Specifies the value to compare the attribute against
-  using the specified operation. This can be a literal value, another attribute,
-  or any [Nomad interpolated
-  values](/nomad/docs/runtime/interpolation#interpreted_node_vars).
+- `value` `(string: <required>)` - Specifies the value to compare the attribute
+  against using the specified operation. This can be a literal value, another
+  attribute, or any [Nomad interpolated
+  values](/nomad/docs/runtime/interpolation#interpreted_node_vars).  The `value`
+  field is required.
 
 - `weight` `(integer: 50)` - Specifies a weight for the affinity. The weight is used
   during scoring and must be an integer between -100 to 100. Negative weights act as

website/content/docs/job-specification/constraint.mdx

@@ -96,7 +96,9 @@ all groups (and tasks) in the job.
 - `value` `(string: "")` - Specifies the value to compare the attribute against
   using the specified operation. This can be a literal value, another attribute,
   or any [Nomad interpolated
-  values](/nomad/docs/runtime/interpolation#interpreted_node_vars).
+  values](/nomad/docs/runtime/interpolation#interpreted_node_vars). The value
+  field is required except for when using the `is_set`, `is_not_set`,
+  `distinct_hosts`, or `distinct_property` operators.
 
 ### `operator` Values