chore: Sign request support multiple documents (#826)

Author: congminh1254

docs/usage/sign_requests.md

@@ -20,7 +20,7 @@ Create Sign Request
 ------------------------
 
 The [`client.create_sign_request(files, signers, parent_folder_id, prefill_tags=None, are_reminders_enabled=None, are_text_signatures_enabled=None, days_valid=None, email_message=None, email_subject=None, external_id=None, is_document_preparation_needed=None, redirect_url=None, declined_redirect_url=None)`][create-sign-request]
-method will create a Sign Request. You need to provide at least one file (from which the signing document will be created) and at least one signer to receive the Sign Request.
+method will create a Sign Request. You need to provide at least one file and up to 10 files (from which the signing document will be created) with at least one signer to receive the Sign Request.
 
 <!-- sample post_sign_requests -->
 ```python

test/unit/client/test_client.py

@@ -1692,7 +1692,11 @@ def test_create_sign_request(mock_client, mock_box_session, mock_sign_request_re
         'id': '12345',
         'type': 'file'
     }
-    files = [source_file]
+    source_file2 = {
+        'id': '34567',
+        'type': 'file'
+    }
+    files = [source_file, source_file2]
 
     signer = {
         'email': '[email protected]'
@@ -1705,6 +1709,10 @@ def test_create_sign_request(mock_client, mock_box_session, mock_sign_request_re
             {
                 'id': source_file['id'],
                 'type': source_file['type']
+            },
+            {
+                'id': source_file2['id'],
+                'type': source_file2['type']
             }
         ],
         'signers': [