spec: all-zeroes special case is for relay IDs, not cell digests

tor-spec.txt Section 5.3 (Creating circuits) says:

   As special cases, if the extend cell includes a digest of
   all zeroes, or asks to extend back to the relay that sent the extend

The implementation uses "digest" internally to refer to relay ID hashes, while tor-spec.txt primarily uses it to refer to the truncated running SHA-1 hash that relays use to "recognize" a cell as targeting them.

arma says this probably intends to refer to relay ID hashes, and pointed to commit e34727a65c411a6260f3960ff8a753088e287565.

We should reword this text to make it clear it refers to relay ID hashes.