- Fixed bug in enhanced constraint-attachment logic introduced in

- Fixed bug in enhanced constraint-attachment logic introduced in
:ticket:`3341` where in the unusual case of a constraint that refers
to a mixture of :class:`.Column` objects and string column names
at the same time, the auto-attach-on-column-attach logic will be
skipped; for the constraint to be auto-attached in this case,
all columns must be assembled on the target table up front.
Added a new section to the migration document regarding the
original feature as well as this change.
fixes #3411