docs: update users guide for ev_volume_integrate_mat
docs: update users guide for ev_volume_integrate_mat

sfepy/sfepy,rc/sfepy,BubuLK/sfepy,vlukes/sfepy,BubuLK/sfepy,vlukes/sfepy,BubuLK/sfepy,sfepy/sfepy,rc/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy