doc: add missing assert return types

This commit documents the return types for assert.rejects() and
assert.doesNotReject().

PR-URL: #57219
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: cjihrig

doc/api/assert.md

@@ -1097,6 +1097,7 @@ added: v10.0.0
 * `asyncFn` {Function|Promise}
 * `error` {RegExp|Function}
 * `message` {string}
+* Returns: {Promise}
 
 Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately
 calls the function and awaits the returned promise to complete. It will then
@@ -2041,6 +2042,7 @@ added: v10.0.0
 * `asyncFn` {Function|Promise}
 * `error` {RegExp|Function|Object|Error}
 * `message` {string}
+* Returns: {Promise}
 
 Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately
 calls the function and awaits the returned promise to complete. It will then