Clarify a bit the guideline on omitting braces, including more examples (NFC)

Like most readability rules, it isn't absolute and there is a matter of taste
to it. I think more recent part of the project may be more consistent in the
current application of the guideline. I suspect sources like
mlir/lib/Dialect/StandardOps/IR/Ops.cpp may be examples of this at the moment.

Differential Revision: https://reviews.llvm.org/D82594