res_fax: check_modem_rate() returned incorrect rate for V.27

Added some text to UPGRADES.txt about the V.27 mode rate changes in r405656.

(issue ASTERISK-22790)
Reported by: Paolo Compagnini






git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@405692 f38db490-d61c-443f-a65b-d21fe96a405b