Advertise into_pages() for pageable functions

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

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

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