docs: update description of IGA field definition
docs: update description of IGA field definition

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