jr:constraintMsg description references jr:requiredMsg="message" example

[tiritea]

under https://opendatakit.github.io/xforms-spec/#bind-attributes:

jr:constraintMsg | Specifies the custom message to be displayed when the constraint is violated. Value can be string literal (jr:requiredMsg="message") or a translation function call (jr:requiredMsg="jr:itext('id')").

'nuf said :-)