Improve caching documentation.
Improve caching documentation.

It was a little weak, and folk need enough detail to reason about
performance.

Change-Id: I67f81ab5a206b3d42dd1e4e1eb52af88ac1a3adc

varunarya10/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,eantyshev/diskimage-builder,rdo-management/diskimage-builder,citrix-openstack-build/diskimage-builder,NaohiroTamura/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,citrix-openstack-build/diskimage-builder,JioCloud/diskimage-builder,rdo-management/diskimage-builder,JioCloud/diskimage-builder,eantyshev/diskimage-builder,varunarya10/diskimage-builder,switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,NaohiroTamura/diskimage-builder