=act #15361 Require non-abstract actor class (earlier)

* It was detected previously as well when trying to find constructor,
  but with a perhaps confusing error message

(cherry picked from commit 15e964b13e91962474c7907787c37121aae8fd4e)
=doc #13965 Fix wrong code sample for BalancingPool

(cherry picked from commit 24ce0605ac954d516a52d9ce14624414c1c79933)
Merge pull request #15464 from akka/wip-13965-15361-master-patriknw

#13965 #15361 cherry picks to master (for validation)