doc: Add note about our preference for scoped enumerations ("enum class") #12800

  38 | @@ -39,6 +39,7 @@ code.
  39 |    - `++i` is preferred over `i++`.
  40 |    - `nullptr` is preferred over `NULL` or `(void*)0`.
  41 |    - `static_assert` is preferred over `assert` where possible. Generally; compile-time checking is preferred over run-time checking.
  42 | +  - `enum class` is preferred over `enum` where possible. Scoped enumerations avoid three potential pitfalls/problems with traditional C++ enumerations: 1.) implicit conversions to int, 2.) name clashes due to enumerators being exported to the surrounding scope, and 3.) inability to specify the underlying type of the enum.

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