05f9770 doc: Clarify developer notes about constant naming (Russell Yanofsky)

Pull request description:

  I'm pretty sure developer notes were intended to say constants should be upper case and variables should be lower case, but right now they are ambiguous about whether to write:

  ```c++
  extern const int SYMBOL;
  ```

  or:

  ```c++
  extern const int g_symbol;
  ```

  First convention above is better than the second convention because it tells you without having to look anything up that the value of `SYMBOL` won't change at runtime. Also I haven't seen other c++ projects using the second convention.

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