Extended properties are essential for good documentation, yes, but if you're adding them to the database itself (after the fact) you have a world of hurt just waiting for you.

Why in the world wouldn't you add this information to the database modelling tool used to design the database and create the build script for it?

That way the extended properties are A) properly maintained in a central "golden authority" and B) ALWAYS placed in the database as part of the creation script/alter scripts.

Much safer, much more consistent, and never subject to "accidents" from hasty editing decisions.