improve docstrings of MRLCBCOperator, ShiftedPeriodicOperator
improve docstrings of MRLCBCOperator, ShiftedPeriodicOperator

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