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

[anemet]

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.

[anemet]

Hal, are you OK with this?

[hfinkel]

LGTM

docs/LangRef.rst

 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ``noalias`` and ``alias.scope`` metadata provide the ability to specify generic
 noalias memory-access sets. This means that some collection of memory access
 instructions (loads, stores, memory-accessing calls, etc.) that carry
 ``noalias`` metadata can specifically be specified not to alias with some other
 collection of memory access instructions that carry ``alias.scope`` metadata.
 Each type of metadata specifies a list of scopes where each scope has an id and
-a domain. When evaluating an aliasing query, if for some domain, the set
+a domain.
+
+When evaluating an aliasing query, if for some domain, the set
 of scopes with that domain in one instruction's ``alias.scope`` list is a
 subset of (or equal to) the set of scopes for that domain in another
 instruction's ``noalias`` list, then the two memory accesses are assumed not to
 alias.
 
+Because scopes in one domain don't affect scopes in other domains, separate
+domains can be used to compose multiple independent noalias sets.  This is
+used for example during inlining.  As the noalias function parameters are
+turned into noalias scope metadata, a new domain is used every time the
+function is inlined.
+
 The metadata identifying each domain is itself a list containing one or two
 entries. The first entry is the name of the domain. Note that if the name is a
 string then it can be combined across functions and translation units. A
 self-reference can be used to create globally unique domain names. A
 descriptive string may optionally be provided as a second list entry.
 
 The metadata identifying each scope is also itself a list containing two or
 three entries. The first entry is the name of the scope. Note that if the name