Small edit to the manual to generalise the form of the multiphase stress term.
Small edit to the manual to generalise the form of the multiphase stress term.

rjferrier/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jrper/fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,rjferrier/fluidity