message: Adding PRF Set section for the wire formats doc. Not very interesting, the format is just as defined by the RFCs for HMAC, AES-CMAC, and HKDF.
diff: <!--*
  # Document freshness: For more information, see go/fresh-source.
  -freshness: { owner: 'sschmieg' reviewed: '2020-01-21' }
  +freshness: { owner: 'sschmieg' reviewed: '2020-08-20' }
  *-->
  This is a description of Tink's wire format for keys and primitive output. The
  @@ -142,6 +142,13 @@ have the exact same format as the AEAD primitive corresponding to the `DEK`.
  Tink follows the corresponding RFCs.
  +### PRF Set
  +
  +Tink follows the corresponding RFCs. Note that for PRF Set the key type differs
  +from the MAC key type of the same algorithm by not including the outputlength.
  +PRF Set keys have to have a RAW output prefix type, as the key ID handling is
  +done by the user. This ensures the output is actually a PRF.
  +
  ### Hybrid Encryption
  Hybrid Encryption uses a Key Encryption Message (KEM) and a Data Encryption
  