Add migration guide mention

diff --git a/site/content/docs/5.1/migration.md b/site/content/docs/5.1/migration.md
index f499f4dde96926f6eda8921b829a56f47abff8b1..81968ee876a87077459dca02d095ff60b7e21dcc 100644 (file)
--- a/site/content/docs/5.1/migration.md
+++ b/site/content/docs/5.1/migration.md
@@ -58,6 +58,8 @@ Your custom Bootstrap CSS builds should now look something like this with a sepa
 - **Introduced new `$enable-container-classes` option.** Now when opting into the experimental CSS Grid layout, `.container-*` classes will still be compiled, unless this option is set to `false`.
 - **Thicker table dividers are now opt-in.** We've removed the thicker and more difficult to override border between table groups and moved it to an optional class you can apply, `.table-group-divider`. [See the table docs for an example.]({{< docsref "/content/tables#table-group-dividers" >}})
 
+- **Popovers and tooltips now use CSS variables.** Both components have been updated to use CSS variables on their base classes, `.popover` and `.tooltip`. Some CSS variables have been updated from their Sass counterparts to reduce the number of variables. As a result, three variables have been deprecated in this release: `$popover-arrow-color`, `$popover-arrow-outer-color`, and `$tooltip-arrow-color`.
+
 ## Dependencies
 
 - Dropped jQuery.