[Docs] Try to clarify the concept of domains for noalias scope

Summary:
This tries to anchor down the concept of domains a bit better.  I had
trouble initially relating this to anything.  Also talking to David
Majnemer on IRC suggested that I wasn't the only one.

Reviewers: hfinkel

Subscribers: llvm-commits, majnemer

Differential Revision: http://reviews.llvm.org/D18799

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267647 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed