fix: Add documentation for FetchEvent, FetchEvent.prototype.respondWith, and FetchEvent.prototype.waitUntil

Author: Jake Champion

documentation/docs/globals/FetchEvent/FetchEvent.mdx

@@ -0,0 +1,28 @@
+---
+hide_title: false
+hide_table_of_contents: false
+pagination_next: null
+pagination_prev: null
+---
+# FetchEvent
+
+This is the event type for `fetch` events. It contains information about the fetch, including the request and how the receiver will treat the response. 
+It provides the [`event.respondWith()`](./prototype/respondWith.mdx) method, which allows us to provide a response to this fetch.
+
+## Instance properties
+
+- `FetchEvent.request` _**readonly**_
+    - : The `Request` that was recieved by the application.
+- `FetchEvent.client` _**readonly**_
+    - : Information about the downstream client that made the request.
+    - `FetchEvent.client.address` _**readonly**_
+        - : A string representation of the IPv4 or IPv6 address of the downstream client.
+    - `FetchEvent.client.geo` _**readonly**_
+        - : A [geolocation dictionary](../../fastly:geolocation/getGeolocationForIpAddress.mdx) corresponding to the IP address of the downstream client. 
+
+## Instance methods
+
+- [`FetchEvent.respondWith()`](./prototype/respondWith.mdx)
+    - : Provide (a promise for) a response for this request.
+- [`FetchEvent.waitUntil()`](./prototype/waitUntil.mdx)
+    - : Extends the lifetime of the event. Used to notify the host environment of tasks that extend beyond the returning of a response, such as streaming and caching.

documentation/docs/globals/FetchEvent/prototype/respondWith.mdx

@@ -0,0 +1,25 @@
+---
+hide_title: false
+hide_table_of_contents: false
+pagination_next: null
+pagination_prev: null
+---
+# FetchEvent.respondWith()
+
+The **`respondWith()`** method allows you to provide a promise for a [`Response`](../../Response/Response.mdx) to send back to the client which made the incoming request to your application.
+
+## Syntax
+
+```js
+respondWith(response)
+```
+
+### Parameters
+
+- `response`
+  - : A [`Response`](../../Response/Response.mdx) or a [`Promise`](../../Promise/Promise.mdx) that resolves to a
+    [`Response`](../../Response/Response.mdx). Otherwise, a network error is returned to Fetch.
+
+### Return value
+
+None `undefined`.

documentation/docs/globals/FetchEvent/prototype/waitUntil.mdx

@@ -0,0 +1,29 @@
+---
+hide_title: false
+hide_table_of_contents: false
+pagination_next: null
+pagination_prev: null
+---
+# FetchEvent.waitUntil()
+
+
+The **`waitUntil()`** method tells the host environment that work is ongoing until the promise settles, and it shouldn't terminate
+the application if it wants that work to complete.
+
+The `waitUntil()` method must be initially called within the event callback,
+but after that it can be called multiple times, until all the promises passed to it
+settle.
+
+## Syntax
+
+```js
+waitUntil(promise)
+```
+
+### Parameters
+
+A [`Promise`](../../Promise/Promise.mdx).
+
+### Return value
+
+None `undefined`.