The following has been included in section C.2:

   The rules governing the use of the R-flag depend on the GIMPS message
   type.  It must always be set in Query messages (these always elicit a
   Response), and never in Confirm, Data or Error messages.  It is
   optional in a MA-Hello; if set, another MA-Hello is sent in reply.
   It is optional in a Response (but must be set if the Response
   contains a Responder cookie); if set, a Confirm is sent in reply.

The text has been updated accordingly in section 5.2.1 in the top level
description of each message, and the Responder cookie in the Confirm marked as
optional in the ABNF.