[docs] A slight tweak to the intro for the golden rule in the coding
standards.

It claims the document intentionally doesn't give fixed standards for
brace placement or spacing, and then the document goes on to do
precisely that in several places. Instead, try to highlight that even
these rules are simply *guidance* which may be trumped by some other
circumstance or the local conventions of code.

I'm not trying to change the thrust of this part of the document, and if
folks think this does so, I'm happy to re-wordsmith it. I just don't
want it to be so self-contradicting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202495 91177308-0d34-0410-b5e6-96231b3b80d8