STL: Consider new conventions for preprocessor comments

We have a long-standing convention of commenting all preprocessor directives. This is extremely useful when preprocessor-controlled code is large and/or nested, because the comments clearly identify region boundaries. (This is even more important now that clang-format indents all preprocessor directives at 0.) Here's an example with large regions:

https://github.com/microsoft/STL/blob/1d39dfac9929c0d3c405edf6bc7eff26bbe2a26b/stl/inc/functional#L1228-L1250

And nested regions:

https://github.com/microsoft/STL/blob/1d39dfac9929c0d3c405edf6bc7eff26bbe2a26b/stl/inc/xstring#L295-L306

But when preprocessor-controlled code is small (down to a single line) and non-nested, it starts looking silly, as @SuperWig observed in https://github.com/microsoft/STL/issues/340#issuecomment-559236933 :

https://github.com/microsoft/STL/blob/1d39dfac9929c0d3c405edf6bc7eff26bbe2a26b/stl/inc/functional#L866-L870

In newer code, we sometimes use "arrow comments" to make regions even more visible, which is an improvement for large and/or nested scenarios as usual:

https://github.com/microsoft/STL/blob/1d39dfac9929c0d3c405edf6bc7eff26bbe2a26b/stl/inc/tuple#L259-L284

But similarly, it is generally silly for small non-nested scenarios (at least with simple conditions; the comments can be useful for complicated conditions even if the controlled code is small):

https://github.com/microsoft/STL/blob/1d39dfac9929c0d3c405edf6bc7eff26bbe2a26b/stl/inc/vector#L691-L695

We should consider developing a simple set of rules for contributors to follow. Example suggestion:

• All #else and #endif directives should have arrow comments,
• Except when the condition is simple, all of the controlled regions are a single line, and there is no nearby nesting.

That would require a bit of human judgement (what is "simple", what is "nearby"), but not very much.

Review the examples linked in stl/inc/functional, stl/inc/xstring, stl/inc/tuple, and stl/inc/vector, along with issue #340's comment. Compare the large, nested, and small preprocessor regions, then determine whether a consistent contributor convention can be agreed on; done means the convention is settled and recorded in the appropriate contributor guidance.