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=.

Clarify use of MsgStream macros

See merge request !1

  - Minor updates: we're now using c++14.  Add note about implicit fallthrough
   warnings with gcc7.  Add rule to use std::abs().