doc: add docs for AssertionError

Fixes: https://github.com/nodejs/node/issues/12274
PR-URL: https://github.com/nodejs/node/pull/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>

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