Add PhpDocs for MgdSchema methods

[bergie]

Currently only MgdSchema methods that have documentation available as reflectable PhpDocs are:

public __construct([$guid_or_id])
    Creates new empty instance or fetch object's data from storage if argument is of integer or string type. In latter case valid guid is expected. Throws midgard_error_exception on failure
public get_by_guid($guid)
    Get object which is identified by given guid

It would be important to have them all contain good documentation.

(copied from http://trac.midgard-project.org/ticket/1499)