Since new headergen is now the default for building LLVM-libc, the docs
need to be updated to reflect that. While I was editing those docs, I
took a quick pass at updating other out-of-date pages.
In the document on undefined behavior, I noted that writing down your
decisions is very important. This document contains all the information
for compile flags and undefined behavior for our printf.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D158311
