CodingStandards - add example of folly::assume_unreachable
CodingStandards - add example of folly::assume_unreachable

Add example of how to handle 'warning:: control reaches end of
non-void function' with fully-covered switch statements.

Change-Id: I22045d8ec5df2ede8cff25e389d15d7dec32159b
Reviewed-on: http://review.couchbase.org/108580
Reviewed-by: Richard de Mellow <3b8534f7ae49c77465cd6f1b512f27ec39f3eddd@couchbase.com>
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine