Minor addition to docs

[krinsman]

Suggested minor addition to the docs to prevent other people from making the same mistake I made.

I know this is fairly obvious in hindsight, but better explicit than implicit I guess. I genuinely did spend around 30-60 minutes puzzled about how to fix this (since I had assumed incorrectly that the default behavior was that the value of proposal.value was accepted automatically as long as no errors were raised by the decorating function, but really functions with @validate decorators seem to be setters as much as anything else).

Again, my fault for not reading the original docs closely enough. I thought it might be helpful to explain the examples a little bit, is all.

[krinsman]

As far as I can tell the reason why this fails one of the Travis tests doesn't relate to the changes I made, since I made no changes to the code, just to the docs, but the failed Travis test was because of one of the lines of code.

[rmorshea]

Yeah, this isn't you. A downstream project is failing.

[rmorshea]

BTW just added #530 to track a 4.3.x release.