Document what the instances are used for, fixes #373.
Document what the instances are used for, fixes #373.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake