ADT: Document advantages of SmallVector<T,0> over std::vector

In light of the recent changes to SmallVector in r335421, r337514, and
r337820, document its advantages over std::vector (see r175906 and
r266909).

Also add a release note.

https://reviews.llvm.org/D49748

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338071 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed