[docs] Slight reword for precision.

The new wording cannot be construed as suggesting the use of
SmallVectorImpl<T> as e.g. a class member (just because the class
happens to be in an interface).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177778 91177308-0d34-0410-b5e6-96231b3b80d8