Added a footnote to the documentation for objc_storeStrong that makes it clear

that a __strong object of block type is a valid argument to objc_storeStrong but
that an objc_retain and not an objc_retainBlock will be emitted.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175838 91177308-0d34-0410-b5e6-96231b3b80d8