Added clearification in docs for PAGE_SAVE_AS

I discussed this option in the IRC channel, because I don't think it's
very clear. When I first read the list of options I was under the
impression that Pelican could do rewriting of urls. I discovered that
this isn't the case, so I discusses this in the IRC channel and got the
encouragement to submit a pull request.
Merge pull request #765 from jeredboxman/master

Added clearification in docs for PAGE_SAVE_AS