docs: update developer guide - describe new Term attributes
docs: update developer guide - describe new Term attributes

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