[squash] add docs

doc/api/async_hooks.md

@@ -203,13 +203,16 @@ UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST,
 RANDOMBYTESREQUEST, TLSWRAP
 ```
 
+There is also the `PROMISE` resource type, which is used to track `Promise`
+instances and asynchronous work scheduled by them.
+
 Users are be able to define their own `type` when using the public embedder API.
 
 *Note:* It is possible to have type name collisions. Embedders are encouraged
 to use a unique prefixes, such as the npm package name, to prevent collisions
 when listening to the hooks.
 
-###### `triggerid`
+###### `triggerId`
 
 `triggerId` is the `asyncId` of the resource that caused (or "triggered") the
 new resource to initialize and that caused `init` to call. This is different
@@ -258,7 +261,13 @@ considered public, but using the Embedder API users can provide and document
 their own resource objects. Such as resource object could for example contain
 the SQL query being executed.
 
-*Note:* In some cases the resource object is reused for performance reasons,
+In the case of Promises, the `resource` object will have `promise` property
+that refers to the Promise that is being initialized, and a `parentId` property
+that equals the `asyncId` of a parent Promise, if there is one, and
+`undefined` otherwise. For example, in the case of `b = a.then(handler)`,
+`a` is considered a parent Promise of `b`.
+
+*Note*: In some cases the resource object is reused for performance reasons,
 it is thus not safe to use it as a key in a `WeakMap` or add properties to it.
 
 ###### asynchronous context example