updating iter_content docstring to match functionality
diff --git a/requests/models.py b/requests/models.py
index 50b7f5da91..7d5d8b5eca 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -657,6 +657,12 @@ def iter_content(self, chunk_size=1, decode_unicode=False):
         read into memory.  This is not necessarily the length of each item
         returned as decoding can take place.
 
+        chunk_size must be of type int or None. A value of None will
+        function differently depending on the value of `stream`.
+        stream=True will read data as it arrives in whatever size the
+        chunks are recieved. If stream=False, data is returned as
+        a single chunk.
+
         If decode_unicode is True, content will be decoded using the best
         available encoding based on the response.
         """

Sorry to be bombarding you guys with PRs. This one is pretty minor but explains what I would say was non-obvious behaviour of the function before PR #3368 and PR #3365.
