docs: describe dw_s_dot_mgrad_s term implementation in developer guide
docs: describe dw_s_dot_mgrad_s term implementation in developer guide

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