update docs (dev guide: rewrite section on implementing new terms)
update docs (dev guide: rewrite section on implementing new terms)

BubuLK/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,rc/sfepy,vlukes/sfepy,BubuLK/sfepy,lokik/sfepy,sfepy/sfepy,vlukes/sfepy,RexFuzzle/sfepy,lokik/sfepy,rc/sfepy,sfepy/sfepy,BubuLK/sfepy,lokik/sfepy,RexFuzzle/sfepy,lokik/sfepy