Clarify behaviour of read_until/async_read_until with respect to leftover

data in the streambuf. Use separate brief descriptions for function groups.
Add some extra notes to the io_service documentation.


[SVN r46950]

libs/asio

@@ -1 +1 @@
-Subproject commit 4123435aa21d54fbc2901406e97adaaee2633eba
+Subproject commit e14e83141391f9f388ef2daf8b2fa48f759a85aa