Link to target patterns from labels section
Fixes
@@ -274,7 +274,9 @@ app_binary
looks for <code>//a/b/c</code> in the external repository itself.
This is especially relevant when writing rules in the main
repository that refer to targets in the main repository, and will be
- used from external repositories.
+ used from external repositories. For information about the different ways
+ you can refer to targets, see
+ [target patterns](guide.html#specifying-targets-to-build).
</p>
