motivate 6.3 product tokens

Hello sirs, I thought it might be useful to add motivation on part one
section 6.3

http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-15#section-6.3

For example, "to assist in automatic detection of the applications' version
levels." That was my thoughts after a few reads.

Respectfully,

Dale Anderson