Document individual expand wildcard values (#601)

Signed-off-by: Thomas Farr <[email protected]>

spec/schemas/_common.yaml

@@ -406,13 +406,22 @@ components:
           items:
             $ref: '#/components/schemas/ExpandWildcard'
     ExpandWildcard:
-      type: string
-      enum:
-        - all
-        - closed
-        - hidden
-        - none
-        - open
+      oneOf:
+        - type: string
+          const: all
+          description: Match any index, including hidden ones.
+        - type: string
+          const: closed
+          description: Match closed, non-hidden indices.
+        - type: string
+          const: hidden
+          description: Match hidden indices. Must be combined with open, closed, or both.
+        - type: string
+          const: none
+          description: Wildcard expressions are not accepted.
+        - type: string
+          const: open
+          description: Match open, non-hidden indices.
     VersionString:
       type: string
     SearchType: