Docs: elaborate on output assertion requirement and benefits in consistent-output rule doc (#123)

Author: bmish

docs/rules/consistent-output.md

@@ -2,6 +2,10 @@
 
 When writing tests for a fixable rule with `RuleTester`, you can assert the autofix output of your test cases. However, it can be easy to forget to assert the output of a particular test case.
 
+[As of ESLint 7](https://eslint.org/docs/user-guide/migrating-to-7.0.0#additional-validation-added-to-the-ruletester-class), test cases that trigger an autofix are required to provide the `output` property.
+
+Even test that do not trigger an autofix can benefit from asserting that they have no autofix using `output: null`.
+
 ## Rule Details
 
 This rule aims to ensure that if any invalid test cases have output assertions, then all test cases have output assertions.
@@ -48,7 +52,7 @@ new RuleTester().run('example-rule', rule, {
     },
     {
       code: 'foo',
-      output: null,
+      output: null, // asserts that there is no autofix
       errors: ['baz']
     }
   ]