DOC: Improve prose and refer to JSON about instance expressions
DOC: Improve prose and refer to JSON about instance expressions

Similar to commit c24539795d5b34d7eaadc8ac18c51c70fdc5ca23, which
concerned the problem domain integrator_chains.

fmrchallenge/fmrbenchmark,fmrchallenge/fmrbenchmark,fmrchallenge/fmrbenchmark