[Docs] Clarify the brace policy for if/else/loop statements (#164570)
Without this patch, the five-paragraph section is unclear about
exactly when to use and not to use braces. Specifically, the first
paragraph suggests omitting braces, and then subsequent paragraphs
carve out exceptions. At the end, it's unclear what situations remain
for omitting braces.
This patch overhauls the text for readability. Specifically, it first
describes when to omit braces and then lists cases where we should
retain braces.