Clarify javadoc for `PageableHandlerMethodArgumentResolverSupport#setFallbackPageable`

[hgjd]

The javadoc for the setFallbackPageable method of PageableHandlerMethodArgumentResolverSupport is unclear in two places.

1. in case no {@link PageableDefault} or {@link PageableDefault} (the latter only supported in legacy mode)
   Both options are the same.
2. If you set this to {@literal Optional#empty()} The method only accepts a non-null Pageable, so it is unclear how to set this to Optional.empty().

Unfortunately I can't propose a fix, because it is not obvious to me what the originally intended message was.

[mp911de]

PageableDefaults was already removed with Spring Data 1.8. We also introduced Pageable.unpaged() with Spring Data 2.0 so the documentation doesn't sense anymore. Let me rewrite the documentation.