Update repository specification with info about the 'pub_display_url' field (#4193)

doc/repository-spec-v2.md

diff --git a/doc/repository-spec-v2.md b/doc/repository-spec-v2.md
index 0a1a1c4..592b0f8 100644
--- a/doc/repository-spec-v2.md
+++ b/doc/repository-spec-v2.md
@@ -429,6 +429,11 @@
 `dart pub` client will exclusively rely on `affected[].versions`, and disregard
 `affected[].ranges`.
 
+In the `database_specific` field of a security advisory the `dart pub` client
+will lookup the property called `pub_display_url`. This property is optional. It
+is intended to be used by the client to display a reference url when reporting
+security advisories.
+
 The `advisoriesUpdated` property is the most recent timestamp of when the result
 from this end-point for this package changed. This can be used for caching
 purposes.