PROTOCOL.md: update message frame maximum data length.

Update maximum data length in a message frame and reserved
most significant byte to be a reserved most significant bit.

Signed-off-by: Krisztian Litkey <krisztian.litkey@intel.com>

PROTOCOL.md

@@ -42,9 +42,9 @@ use is defined by the message type.
 
 The Data Length field represents the number of bytes in the Data field. The
 total frame size will always be Data Length + 10 bytes. The maximum data length
-is 4MB and any larger size should be rejected. Due to the maximum data size
-being less than 16MB, the first frame byte should always be zero. This first
-byte should be considered reserved for future use.
+is limited to the largest positive 32-bit signed integer. Therefore the most
+signficant bit should always be zero. This bit should be considered reserved
+for future use.
 
 The Stream ID must be odd for client initiated streams and even for server
 initiated streams. Server initiated streams are not currently supported.