Bug 1918423: Missing function generate_simple_struct_class in soap webservice...

Bug 1918423: Missing function generate_simple_struct_class in soap webservice in /webservice/soap/locallib.php

- Re-ordered to match moodle_31 ordering where we have copied code.
- Additional documentation copied over from moodle_31

The following functions live in webservice/lib.php in moodle_31
but got ported into soap/locallib.php in the past so I've left them as
is but re-ordered them to match the original code.
- init_service_class
- service_class_method_body
- get_phpdoc_type
- get_virtual_method_code
- init_service_class
Change-Id: Ieb204161d170fe601169bbb679a7f0ef3e8031f0