Improve iids example and error message

Reviewer suggestions.

app/graphql/resolvers/concerns/issue_resolver_arguments.rb

@@ -11,7 +11,7 @@ module IssueResolverArguments
              description: 'IID of the issue. For example, "1".'
     argument :iids, [GraphQL::STRING_TYPE],
              required: false,
-             description: 'List of IIDs of issues. For example, [1, 2].'
+             description: 'List of IIDs of issues. For example, ["1", "2"].'
     argument :label_name, [GraphQL::STRING_TYPE, null: true],
              required: false,
              description: 'Labels applied to this issue.'

lib/gitlab/graphql/docs/helper.rb

@@ -150,7 +150,7 @@ module Gitlab
             input = inputs.first
             name = t[:name]
 
-            raise ViolatedAssumption, "Expected one input field to #{name}" unless inputs.one?
+            raise ViolatedAssumption, "Expected exactly 1 input field named #{name}. Found #{inputs.count} instead." unless inputs.one?
             raise ViolatedAssumption, "Expected the input of #{name} to be named 'input'" if input[:name] != 'input'
 
             input_type_name = input[:type][:name]