V1model notes on register extern (#817)
V1model notes on register extern (#817)

* Add runtime_CLI docs on table_add and a few other commands

* Additional implementation notes on registers in simple_switch
Some of this material comes from this comment by Antonin Bas:
https://github.com/p4lang/p4c/issues/2055#issuecomment-541863460

* Add missing backquote markup

p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model