documentation on multi-dim indexing in Reference Guide, User's Manual

[mitzimorris]

Summary:

cf discussion here: https://discourse.mc-stan.org/t/possible-error-in-user-guide-and-reference-manual/19625/4

Description:

examples in Reference Manual and User's Guide are impossible to follow, especially in the latter. spell it out.

Additional Information:

Provide any additional information here.

Current Version:

v2.18.0