Correction to hasNext and hasPrevious Javadoc

[njr-11]

Page.hasNext and hasPrevious say they are a way of knowing
that {@link #nextPageRequest()} will definitely not return {@code null}.
that {@link #previousPageRequest()} will not return {@code null}

However, nextPageRequest and previousPageRequest are written to never return null. They likely did so at one point but were switched to raise NoSuchElementException rather than returning null. The Javadoc for hasNext and hasPrevious needs to be updated to be consistent with that.

There is no behavior change here, only a correction to the informative statement we are making to the user.

Also, I corrected some indenting in PageRequest but there are no updates to wording in that class.

[KyleAure]

Looks good, this clarifies the expected behavior well.