Add a comment explaining some confusing IOBuf functions

Summary: The len and bytes functions only operate on the current IOBuf, so add some comments explaining it.

Reviewed By: nanshu

Differential Revision: D30501776

fbshipit-source-id: 997dd77db52a3c11722efff8ae83b05e073e1285

folly/python/iobuf.pyi

@@ -25,8 +25,10 @@ class IOBuf(Hashable):
     def is_chained(self) -> bool: ...
     def chain_size(self) -> int: ...
     def chain_count(self) -> int: ...
+    # Note that this gives you the bytes of the current IOBuf, for the whole IOBuf iterate through it or do b''.join(iobuf)
     def __bytes__(self) -> bytes: ...
     def __bool__(self) -> bool: ...
+    # Note that this gives you the size of the current IOBuf, for the whole IOBuf use chain_size
     def __len__(self) -> int: ...
     def __iter__(self) -> Iterator[memoryview]: ...
     def __hash__(self) -> int: ...