Document Kar support for actor deadlock detection

[dgrove-oss]

Kar implementation includes features to help avoid and/or debug common actor-actor deadlock scenarios. For example, the synchronous scenario A-calls-B-calls-A is implicitly supported and is described (somewhat abstractly as indirect re-entrancy) in KAR.md in "Actors: Sessions".

However, undocumented are Kar features that help detect and resolve other likely actor deadlock scenarios. For example, given an actor-based backend supporting concurrent asynchronous requests, the deadlock (A-calls-B while B-calls-A) may be likely. Kar's timeout mechanism combined with rich exception messaging significantly help identify the improper designs that cause such problems.