Document that .serialize() should only be run on forms, and not fieldsets

[mikesherov]

http://bugs.jquery.com/ticket/11594

from @dmethvin:

So other than a form, there is no current use of .serialize() that requires the code to search inside an element for more elements to process.

from @ajpiano:

I don't think adding a note about using it on fieldsets is a terrible idea, so +1 for that. This could perhaps be phased in terms of explaining that "a collection that contains an element that contains form elements is not the same as a collection that contains form elements." Using the word "represent" is probably misleading here.

But in terms of workarounds, I just thought it's worth noting that $("#someFieldset :input").serialize() is probably a wiser move than nested forms, etc, so I wanted to chime in with that.