Clarify label syntax
The distinction between `@//` and `//` labels is subtle and in some cases very important. This PR adds a paragraph to the "Concepts and Terminology" documentation to help clarify.
Updates
Closes
@@ -298,6 +298,18 @@ testdata/input.txt
<code>//my/app/testdata:testdepot.zip</code>.
</p>
+<p>
+ Labels starting with <code>@//</code> are references to the main
+ repository, which will still work even from external repositories.
+ Therefore <code>@//a/b/c</code> is different from
+ <code>//a/b/c</code> when referenced from an external repository.
+ The former refers back to the main repository, while the latter
+ 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.
+</p>
+
<h3 id="lexi">Lexical specification of a label</h3>
<p>
