LangRef: clarify that global declarations can have section and alignment info.

I'm not sure what it means to set a section for a declaration in another
translation unit, but there are some tests in the tree that do it so it seems
to be legal now regardless.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210819 91177308-0d34-0410-b5e6-96231b3b80d8