[prdoc] Support multiple audiences (#3990)

Closes https://github.com/paritytech/polkadot-sdk/issues/3986

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

prdoc/schema_user.json

@@ -98,6 +98,12 @@
     "$defs": {
       "audience": {
         "description": "You may pick one or more audiences and address those users with appropriate documentation, information and warning related to the PR.",
+        "oneOf": [
+          { "$ref": "#/$defs/audience_id" },
+          { "$ref": "#/$defs/audience_ids" }          
+        ]
+      },
+      "audience_id": {
         "oneOf": [
           {"const": "Node Dev",
           "title": "Node Dev",
@@ -116,6 +122,13 @@
           "description": "Anyone using the runtime. This can be a token holder or a dev writing a front end for a chain."}
         ]
       },
+      "audience_ids": {
+        "type": "array",
+        "items": {
+          "$ref": "#/$defs/audience_id"
+        },
+        "minItems": 2
+      },
       "crate": {
         "type": "object",
         "description": "You have the option here to provide a hint about a crate that has changed to help with the publishing of crates.",
@@ -209,7 +222,11 @@
             "description": "Description of the change",
             "type": "string"
           }
-        }
+        },
+        "required": [
+          "audience",
+          "description"
+        ]
       },
       "array_of_strings": {
         "description": "An array of strings that can be empty",