socket: update comment
The socket abstraction no longer handles leading nul bytes.
* message-based buffers for DBus transactions.
*
* The first line (if any) of a SASL exchange sent from a client to a server
- * must be prepended with a null byte. The wrapper handles this internally.
+ * must be prepended with a null byte, which the caller must take care of.
*
* Note that once the first real DBus message was read, you must not use the
* line-helpers, anymore!
