Merge pull request rails#45802 from EduardoGHdez/docs/belongs-to-default

[ci-skip] Docs: Update belongs_to [:default] option section

Author: rafaelfranca

activerecord/lib/active_record/associations.rb

@@ -1768,6 +1768,7 @@ def has_one(name, scope = nil, **options)
         # [:default]
         #   Provide a callable (i.e. proc or lambda) to specify that the association should
         #   be initialized with a particular record before validation.
+        #   Please note that callable won't be executed if the record exists.
         # [:strict_loading]
         #   Enforces strict loading every time the associated record is loaded through this association.
         # [:ensuring_owner_was]