better documentation/discussion of sampler config for user-specified mass matrix

[mitzimorris]

Summary:

update all relevant docs to better explain how to use the save mass matrix when running the sampler.

Description:

see comments from Oct 2019 at end of this thread: https://discourse.mc-stan.org/t/saved-mass-matrix

Additional Information:

Provide any additional information here.

Current Version:

v2.18.0