Update the description of the greeting protocol packet

doc/sphinx/dev_guide/box-protocol.rst

@@ -70,9 +70,10 @@ IPROTO is a binary request/response protocol.
     64                  107              127
 
 The server begins the dialogue by sending a fixed-size (128 bytes) text greeting
-to the client. The first 64 bytes of the greeting contain server version. The
-second 44 bytes contain a base64-encoded random string, to use in authentication
-packet. And it ends with 20 bytes of spaces.
+to the client. The greeting always contains two 64 byte lines of ascii text, each line
+ending with newline character ('\n'). The first line contains the server version and 
+protocol type. The second line contains up to 44 bytes of base64-encoded 
+random string, to use in authentication packet, and ends with up to 23 spaces.
 
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                          Unified packet structure