message: Fix docs parser to handle keras docs
  Most keras files use 4-space indentation for the "Args/Raises/Returns" block. These were rendering as code cells on the website. Switched the doc parser to allow 2-to-4-spaces.
diff: @@ -33,7 +33,7 @@ from google.protobuf.message import Message as ProtoMessage
  from tensorflow.python.util import tf_inspect
  -# A regular expression capturing a python indentifier.
  +# A regular expression capturing a python identifier.
  IDENTIFIER_RE = '[a-zA-Z_][a-zA-Z0-9_]*'
  @@ -507,7 +507,7 @@ def _parse_function_details(docstring):
  pairs = list(_gen_pairs(parts[1:]))
  function_details = []
  - item_re = re.compile(r'^ (\*?\*?\w+):', re.MULTILINE)
  + item_re = re.compile(r'^ ? ?(\*?\*?\w+\s*):\s', re.MULTILINE)
  for keyword, content in pairs:
  content = item_re.split(content)
  