document special material parameters in Material.__init__() docstring
document special material parameters in Material.__init__() docstring

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