[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

llvm-svn: 267647