Explicitly State Range of Return Values for Work-item Functions

It would be helpful to explicitly state the range of legal return values for the work-item functions that return an "id".

It looks like we are doing this for `get_group_id` and `get_sub_group_id`, for example:

> get_sub_group_id returns the subgroup ID which is a number from 0 .. get_num_sub_groups() - 1.
>
> For clEnqueueTask, this returns 0.

We don't say this explicitly for any of the other "id" functions though.

Start with the reference pages for get_group_id, get_sub_group_id, and clEnqueueTask, which are the examples mentioned. Identify the other work-item functions returning an id and document each legal range consistently; done when those pages state the ranges.

Written by the indexing model from the issue text.