Update the LangRef to reflect reality.

At the very least, we have CallInst::setIsNoInline() for adding the
noinline attribute to callsites, and I'm told alwaysinline seems to
work.

Thought of adding "not all attributes are guaranteed to work here". If
someone thinks that would be better (or has a better way of phrasing
that, etc.), happy to add it.

llvm-svn: 300168