Mention "return" -> "thread return"


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175689 91177308-0d34-0410-b5e6-96231b3b80d8
*ahem* Enrico.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175691 91177308-0d34-0410-b5e6-96231b3b80d8