doc: add docs for AssertionError

Fixes: #12274 PR-URL: #14261 Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com> Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com> Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: Tobias Nießen <tniessen@tnie.de> Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Khaidi Chu <i@2333.moe>
nodejs · evanlucas · Aug 15, 2017 · Jul 31, 2017 · Aug 2, 2017 · Sep 28, 2016
commit d3b072276b0e11364bf0a25547ec9c1bc1e4e373
diff --git a/doc/api/errors.md b/doc/api/errors.md
@@ -358,6 +358,18 @@ loop tick.
 System-level errors are generated as augmented `Error` instances, which are
 detailed [here](#errors_system_errors).
 
+## Class: AssertionError
+
+A subclass of `Error` that indicates the failure of an assertion. Such errors
+commonly indicate inequality of actual and expected value.
+
+For example:
+
+```js
+assert.strictEqual(1, 2);
+// AssertionError [ERR_ASSERTION]: 1 === 2
+```
+
 ## Class: RangeError
 
 A subclass of `Error` that indicates that a provided argument was not within the