Clarify function invocation

Clarify description of function (import) invocation, especially

• non-composable functions only support GET, function must be last path segment
• composable functions allow additional path segments, last path segment determines allowed HTTP verbs, e.g. POST if last segment is multi-valued navigation property etc.

See discussion on https://groups.google.com/forum/#!topic/odata-discussion/djoPGQK3ZsE