Add docs for getElementError config option (testing-library#376)

Co-authored-by: davidsa <alec.davidson@adp.com>

Author: aw-davidson

docs/dom-testing-library/api-configuration.md

@@ -21,15 +21,28 @@ Configuration options:
 `testIdAttribute`: The attribute used by [`getByTestId`](api-queries#bytestid)
 and related queries. Defaults to `data-testid`.
 
+`getElementError`: A function that returns the error used when
+[`getBy*`](api-queries#getby) or [`getAllBy*`](api-queries#getallby) fail. Takes
+the error message and container object as arguments.
+
 <!--DOCUSAURUS_CODE_TABS-->
 
 <!--Native-->
 
 ```js
 // setup-tests.js
 import { configure } from '@testing-library/dom'
-
-configure({testIdAttribute: 'data-my-test-id'})
+import serialize from 'my-custom-dom-serializer'
+
+configure({
+ testIdAttribute: 'data-my-test-id',
+ getElementError: (message, container) => {
+ const customMessage = [message, serialize(container.firstChild)].join(
+ '\n\n'
+ )
+ return new Error(customMessage)
+ },
+})
 ```
 
 <!--React-->
@@ -38,7 +51,7 @@ configure({testIdAttribute: 'data-my-test-id'})
 // setup-tests.js
 import { configure } from '@testing-library/react'
 
-configure({testIdAttribute: 'data-my-test-id'})
+configure({ testIdAttribute: 'data-my-test-id' })
 ```
 
 <!--Cypress-->
@@ -47,7 +60,7 @@ configure({testIdAttribute: 'data-my-test-id'})
 // setup-tests.js
 import { configure } from '@testing-library/cypress'
 
-configure({testIdAttribute: 'data-my-test-id'})
+configure({ testIdAttribute: 'data-my-test-id' })
 ```
 
 <!--END_DOCUSAURUS_CODE_TABS-->