C++ coding style is not documented

Ran into this issue when reviewing some recent additions.

Currently the only documentation I could find was that we specify BasedOnStyle: Chromium in .clang-format. This implicitly locks down a lot of things which good, more explicit docs would be good.

The specific issue we ran into was the wabt uses a style that requires curly braces around conditional blocks, but chromium, which inherits from google style allows from single line conditional, and conditional without curlys:
https://google.github.io/styleguide/cppguide.html#Conditionals
In general, curly braces are not required for single-line statements

We should be specific if we want to enforce this style.

Start with the repository's .clang-format and compare its BasedOnStyle: Chromium setting with the linked Chromium C++ style guidance. Document the intended C++ style, including the conditional-brace rule raised in the issue, and consider other implicit choices that should be explicit. Done means contributors and reviewers can find and apply the documented conventions.