start beta 3 migration docs, mention removed var

diff --git a/docs/4.0/migration.md b/docs/4.0/migration.md
index c8c1fd6b41aa26801f284909742c55c51f8bee64..1d18fb1b86b315c12f476e0f31ad3664751d0932 100644 (file)
--- a/docs/4.0/migration.md
+++ b/docs/4.0/migration.md
@@ -6,6 +6,12 @@ group: migration
 toc: true
 ---
 
+## Beta 3 changes
+
+While Beta 2 saw the bulk of our breaking changes during the beta phase, but we still have a few that needed to be addressed in the Beta 3 release. These changes apply if you're updating to Beta 3 from Beta 2 or any older version of Bootstrap.
+
+- Removed the unused `$thumbnail-transition` variable. We weren't transitioning anything, so it was just extra code.
+
 ## Beta 2 changes
 
 While in beta, we aim to have no breaking changes. However, things don't always go as planned. Below are the breaking changes to bear in mind when moving from Beta 1 to Beta 2.