doc: add context.assert docs

When context.assert was added, no docs were added. This commit
adds initial documentation for context.assert because the
snapshot() function requires them.

Refs: nodejs#52860

Author: cjihrig

doc/api/test.md

@@ -3024,6 +3024,24 @@ test('top level test', async (t) => {
 });
 ```
 
+### `context.assert`
+
+<!-- YAML
+added:
+  - v22.2.0
+-->
+
+An object containing assertion methods bound to `context`. The top-level
+functions from the `node:assert` module are exposed here for the purpose of
+creating test plans.
+
+```js
+test('test', (t) => {
+  t.plan(1);
+  t.assert.strictEqual(true, true);
+});
+```
+
 ### `context.diagnostic(message)`
 
 <!-- YAML