CodingStandards: Document std::equal misbehaviour

I should have included this as part of r215986, which worked around this
corner by changing ArrayRef::equals() not to use std::equal.  Alas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215988 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed