A Note element is a textual annotation that can be attached to a set of elements of any other type. The attachment is created separately, using a Notelink connector. Both Note and Notelink are available in any Enterprise Architect diagram, through the 'Common' pages of the Toolbox.
A Note is also called a Comment.
A Constraint is a form of Note, identifying a constraint on other elements. As for a Note, you can connect the Constraint element to other elements using a Notelink connector. This element is just a means of documenting the fact that there are constraints; it has no impact on the other elements. You define the types of constraint in the project reference data, apply them to the element in the element 'Properties' dialog, and manage them through the Responsibilities window.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p.59) states:
A comment gives the ability to attach various remarks to elements. A comment carries no semantic force, but may contain information that is useful to a modeler.
A comment can be owned by any element.