- Allow omitting the =default= clause in a =switch= statement on an =enum=

   that handles all possible values.  Recent compilers will warn if some
   values are not handled, and it's better to get such a diagnosic
   at compile-time rather than at runtime.
 - Clarify avoid-typedef section.
 - Mention preference for =ATH_MSG_= macros.
 - Don't require =override= for destructors.
 - Avoid using =#pragma once=.