docs: use only unified volume/surface terms in users' guide
docs: use only unified volume/surface terms in users' guide

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