[Docs] Clarify the guideline on omitting braces

While working on a clang-format option RemoveBracesLLVM that removes
braces following the guideline, we were unsure about what to do with
the braces of do-while loops. The ratio of using to omitting the
braces is about 4:1 in the llvm-project source, so it will help to
add an example to the guideline.

Also cleans up the original examples including making the nested if
example more targeted on avoiding potential dangling else situations.

Differential Revision: https://reviews.llvm.org/D126512