Warn about HiPE deprecation

References rabbitmq/rabbitmq-website#884

(cherry picked from commit b98de809328d0e1ccafa2bc47de38b4a4a04cb39)

Author: michaelklishin

docs/rabbitmq.conf.example

@@ -460,7 +460,9 @@
 ## NB: Change these only if you understand what you are doing!
 ##
 
-## Explicitly enable/disable hipe compilation.
+## Explicitly enable/disable hipe compilation. This option is deprecated
+## because HiPE supported has been
+## effectively discontinued in Erlang 22.
 ##
 # hipe_compile = false