Adds a doc comment for new_batch_http_request

This comment doesn't actually show up in the generated epydoc, but it
does allow the "make docs" command to complete, and it is nice to have
the explanation of how to use the method in the source repository.

Without this change running "make docs" would have an error:
$> make docs
cd docs; ./build
mkdir -p docs/dyn
python describe.py
Traceback (most recent call last):
  File "describe.py", line 388, in <module>
    document_api(api['name'], api['version'])
  File "describe.py", line 354, in document_api
    service, '%s_%s.' % (name, version), discovery, discovery)
  File "describe.py", line 320, in document_collection_recursive
    html = document_collection(resource, path, root_discovery,
                               discovery)
  File "describe.py", line 302, in document_collection
    params = method_params(doc)
  File "describe.py", line 192, in method_params
    doclines = doc.splitlines()
AttributeError: 'NoneType' object has no attribute 'splitlines'
make: *** [docs] Error 1