Improve docstring for add_element!

[ahojukka5]

add_element!(mesh, elid, eltype, connectivity)
Add an element into the mesh. ´elid´ is the element id, ´eltype´ is the type of the element and ´connectivity´ is the connectivity of the element.

At least eltype needs to be defined more precisely. I guess it's a symbol :Seg2, :Tri3 and so on.