improved documentation for atime.gap

The old documentation for atime.gap seemed to be not very exact and a bit
misleading, especially it lacked information about absolute differences being
used.

* Rewrote the documentation for atime.gap based on the code in modules/dcache-chimera/src/main/java/org/dcache/chimera/namespace/ChimeraNameSpaceProvider.java
  including an example for how it works.