E-BFMI isn't documented

[jgabry]

Following up on https://discourse.mc-stan.org/t/issues-with-e-bfmi-missing-doc-confusing-name-etc/22553.

E-BFMI is not documented anywhere (at least using the search functionality in the user's guide and reference manual there were no results for "BFMI"). The only place I found any mention of the E-BFMI diagnostic is in the CmdStan Guide but it only links to a paper, so that's not even really software documentation (although it's better than what we have elsewhere).

Current Version:

v2.26.0