IPC: Documentation is ambiguous w.r.t buffer size

The description in Section 9.3.7. Large Buffer Support of the PDK documentation
is ambiguous. It currently has the following text:

The maximum size for each message in IPC LLD is 512 bytes. For large buffers, it is recomended to pass a pointer/offset to the buffer in a shared memory location.

512 bytes is the transport size, and the maximum payload/user message size is actually 496 bytes.