Update reference documentation to reflect requirements for usage with spring-native.

[christophstrobl]

Add a section that outlines required steps and limitations when using Spring Data with Graal native image and spring-aot.

[christophstrobl]

Updated information in spring-native (see: spring-attic/spring-native#808).
We should still include a section in the data docs as well, might even be enough to link to the spring-native documentation.