Clarify what the RELAY cell's 'recognized' field is for

Our tor-spec left me pretty mystified what the 'recognized' field actually was.
It discussed what to do when it was zero, but not what the field *was* or what
non-zero meant. Thankfully Roger filled me in over tasty, tasty pizza.

tor-spec.txt

@@ -1443,13 +1443,19 @@ see tor-design.pdf.
    other nodes in the circuit back to the originator. Commands marked
    as either can be sent either by the originator or other nodes.
 
-   The 'recognized' field in any unencrypted relay payload is always set
-   to zero; the 'digest' field is computed as the first four bytes of
-   the running digest of all the bytes that have been destined for
-   this hop of the circuit or originated from this hop of the circuit,
-   seeded from Df or Db respectively (obtained in section 5.2 above),
-   and including this RELAY cell's entire payload (taken with the digest
-   field set to zero).
+   The 'recognized' field is used for a simple indication for if the
+   cell still encrypted or not. When sending cells the unencrypted
+   'recognized' MUST be set to zero. When receiving and decrypting
+   cells the 'recognized' will be zero if we're the endpoint the cell
+   is destined for, and non-zero otherwise.
+
+   When 'recognized' in an unencrypted relay payload is zero the
+   'digest' field is computed as the first four bytes of the running
+   digest of all the bytes that have been destined for this hop of
+   the circuit or originated from this hop of the circuit, seeded from
+   Df or Db respectively (obtained in section 5.2 above), and including
+   this RELAY cell's entire payload (taken with the digest field set to
+   zero).
 
    When the 'recognized' field of a RELAY cell is zero, and the digest
    is correct, the cell is considered "recognized" for the purposes of