Add a caution message about using UUIDs as primary keys

Author: javiereguiluz

components/uid.rst

@@ -344,6 +344,14 @@ entity primary keys::
         // ...
     }
 
+.. caution::
+
+    Using UUIDs as primary keys is usually not recommended for performance reasons:
+    indexes take more space (because UUIDs in binary format take 128 bits instead
+    of 32/64 bits for auto-incremental integers) and the non-sequential nature of
+    UUIDs fragments indexes. UUID v7 is the only variant that solves the fragmentation
+    issue (but the index size issue remains).
+
 When using built-in Doctrine repository methods (e.g. ``findOneBy()``), Doctrine
 knows how to convert these UUID types to build the SQL query
 (e.g. ``->findOneBy(['user' => $user->getUuid()])``). However, when using DQL