Pet-friendly motel - allows pets up to 25 pounds - $25 per night fee for each pet. The carpet was dirty, and there was sand in the corners of the room. The beds were doubles, and the pillows were old and uncomfortable. The dresser drawers were broken. The bathroom walls were dirty. I must say, all the employees were polite and helpful.
The definition of a restraint is something that restricts freedom or prevents someone from doing something. When someone is tied up and prevented from moving, this is an example of restraint. When your budget sets a limit on how much you can spend for Christmas, this is an example of financial restraint.
It appears that constraint is something imposed from without, like laws or morals, but restraint is something a person imposed on her/himself. However, here are definitions of these words from the American Heritage Dictionary*. While there is a distinction between the two words, as in your definition, note that meaning # 4 under restraint, is constraint.. n. 1. The threat or use of force to prevent, restrict, or dictate the action or thought of others.