It was that you could use quoted punctuation to punctuate a sentence, but you shouldn't quote anything that's not part of the quote. For example, you wouldn't say

The first word of that popular birthday song is "happy."

This would suggest that the first line goes "Happy. Birthday to you" whereas you could just finish after the quote: first word is "happy".

Also, compare

He said "hello"?
He said "hello?"
He said "hello?"?

All mean different things, although that last one does look very odd.