[1.7.x] Added a warning about nonexistent FK constraints when unmigrated apps depend on migrated ones.

Thanks NotSqrt for the report; refs #23741.

Backport of f0ff452451 from master

1 files changed, 7 insertions, 2 deletions

diff --git a/docs/topics/migrations.txt b/docs/topics/migrations.txt
index c43b256eff..be8dd6e659 100755
--- a/docs/topics/migrations.txt
+++ b/docs/topics/migrations.txt
@@ -213,8 +213,13 @@ will be.
 
 Be aware, however, that unmigrated apps cannot depend on migrated apps, by the
 very nature of not having migrations. This means that it is not generally
-possible to have an unmigrated app have a ForeignKey or ManyToManyField to
-a migrated app; some cases may work, but it will eventually fail.
+possible to have an unmigrated app have a ``ForeignKey`` or ``ManyToManyField``
+to a migrated app; some cases may work, but it will eventually fail.
+
+.. warning::
+
+    Even if things appear to work with unmigrated apps depending on migrated
+    apps, Django may not generate all the necessary foreign key constraints!
 
 This is particularly apparent if you use swappable models (e.g.
 ``AUTH_USER_MODEL``), as every app that uses swappable models will need