Add comment clarifying that the proto2 benchmark is ugly and temporary.
Add comment clarifying that the proto2 benchmark is ugly and temporary.

tempbottle/upb,tbetbetbe/upb,tbetbetbe/upb,tempbottle/upb,bsn069/upb,bsn069/upb,tbetbetbe/upb,tempbottle/upb,tbetbetbe/upb,bsn069/upb,tbetbetbe/upb,tempbottle/upb,bsn069/upb,tempbottle/upb,bsn069/upb