Add description of eigenvalue_issues heuristic in model_summary() docs

[timwaterhouse]

model_summary() documentation currently lists details on other run heuristics but is missing eigenvalue_issues.

From bbi news:
Added eigenvalue_issues heuristic, triggered when any of the of the eigenvalues <= 0, or if NONMEM needs to force them to be positive. (#216)

[seth127]

Yep, in looking into this, @timwaterhouse and I (and others) discussed this here and I even said

The docs for ?model_summary would explain that this flag can mean a few different things and "the user should check...".

But then I guess I never did that. Oops.