[LangRef] add to description of alloca instruction

As mentioned in D58359, we can explicitly state that the
memory allocated is uninitialized and reading that memory
produces undef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354394 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed