document optional url arg in request

Author: lhorie

docs/request.md

@@ -18,10 +18,11 @@
 
 ### API
 
-`stream = m.request(options)`
+`stream = m.request([url,] options)`
 
 Argument               | Type                              | Required | Description
 ---------------------- | --------------------------------- | -------- | ---
+`url`                  | `String`                          | No       | If present, it's equivalent to having the options `{method: "GET", url: url}`. Values passed to the `options` argument override options set via this shorthand.
 `options.method`       | `String`                          | Yes      | The HTTP method to use. This value should be one of the following: `GET`, `POST`, `PUT`, `PATCH`, `DELETE`, `HEAD` or `OPTIONS`.
 `options.url`          | `String`                          | Yes      | The URL to send the request to. The URL may be either absolute or relative, and it may contain [interpolations](#dynamic-urls).
 `options.data`         | `any`                             | No       | The data to be interpolated into the URL and serialized into the querystring (for GET requests) or body (for other types of requests).

docs/signatures.md

@@ -27,6 +27,14 @@ The **Required** column indicates whether an argument is required or optional. I
 
 ---
 
+### Optional arguments
+
+Function arguments surrounded by square brackets `[ ]` are optional. In the example below, `url` is an optional argument:
+
+`m.request([url,] options)`
+
+---
+
 ### Splats
 
 A splat argument means that if the argument is an array, you can omit the square brackets and have a variable number of arguments in the method instead.