Document "headers" upload parameter in AwsS3 plugin

The AwsS3 plugin appears to already support the "headers" field in
upload parameter, indicating the request headers that should be sent
along with the upload request, so we add it to the documentation.

src/plugins/AwsS3/index.js

@@ -65,7 +65,7 @@ module.exports = class AwsS3 extends Plugin {
       (params.method == null || /^(put|post)$/i.test(params.method))
 
     if (!valid) {
-      const err = new TypeError(`AwsS3: got incorrect result from 'getUploadParameters()' for file '${file.name}', expected an object '{ url, method, fields }'.\nSee https://uppy.io/docs/aws-s3/#getUploadParameters-file for more on the expected format.`)
+      const err = new TypeError(`AwsS3: got incorrect result from 'getUploadParameters()' for file '${file.name}', expected an object '{ url, method, fields, headers }'.\nSee https://uppy.io/docs/aws-s3/#getUploadParameters-file for more on the expected format.`)
       console.error(err)
       throw err
     }

website/src/docs/aws-s3.md

@@ -33,7 +33,7 @@ uppy.use(AwsS3, {
 > Note: When using [uppy-server][uppy-server docs] to sign S3 uploads, do not define this option.
 
 A function returning upload parameters for a file.
-Parameters should be returned as an object, or a Promise for an object, with keys `{ method, url, fields }`.
+Parameters should be returned as an object, or a Promise for an object, with keys `{ method, url, fields, headers }`.
 
 The `method` field is the HTTP method to use for the upload.
 This should be one of `PUT` or `POST`, depending on the type of upload used.
@@ -45,6 +45,8 @@ When using a POST upload with a policy document, this should be the root URL of
 The `fields` field is an object with form fields to send along with the upload request.
 For presigned PUT uploads, this should be empty.
 
+The `headers` field is an object with request headers to send along with the upload request.
+
 ### `timeout: 30 * 1000`
 
 When no upload progress events have been received for this amount of milliseconds, assume the connection has an issue and abort the upload. This is passed through to [XHRUpload](/docs/xhrupload#timeout-30-1000); see its documentation page for details.