Advertise into_pages() for pageable functions

[heaths]

To better advertise that callers can iterate over pages and not just paged items, add documentation to the emitted functions like:

/// <existing summary>
///
/// <existing description, if any>
///
/// The returned `Pager<T>` will iterate over items of type `T` by default.
/// To iterate over each page of type `P`, call `into_pages()` on the returned `Pager<T>` before iterating any items.

Of course, replace T and P with the item type and page types, respectively.