update docstring of convert_mesh.py
update docstring of convert_mesh.py

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