[docs] Clarify that there isn't much to be done other than watch build
bots when using the standard library facilities. The missing pieces here
aren't always in useful discreet chunks.

Fortunately, the missing pieces are few and far between, and we can
emulate most of them in our headers as needed.

Based on feedback from Lang and Dave.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202548 91177308-0d34-0410-b5e6-96231b3b80d8