Improve documentation for creating PermutationGroupElement.

[nthiery]

I had a student today that had a bit of difficulty finding the documentation explaining how to create a permutation group element.

This is actualy well documented, but in the __init__ method of PermutationGroupElement?. Because of this, when typing PermutationGroupElement? the user first gets the documentation of the class itself (which is currently not particularly useful), and only after the useful documentation of the __init__. This also means that the useful doc does not appear in the html documentation.

What about moving the documentation on how to create permutation group elements to the class docstring?

Thanks!