doc: document some more constructors variants

[fingolfin]

For example, right now, we only document SI_matrix( r, c, str ) (which actually is incorrect, it should be SI_matrix( ring, rows, columns, str ). But as a matter of fact, there is also SI_matrix( ring, rows, columns, list_of_elements ). And, like the other constructors, SI_matrix also serves a double-role as a type cast operation: SI_matrix(x) which can e.g. convert a module into a matrix.

Same goes for the other constructors.