Auto merge of #2887 - wuranbo:doc-explain-doublebrackets, r=alexcrichton

doc: manifest.md add explain about double-bracket

Add TOML doc ref to explain double-bracket sections like [[bin]].
It really confused me when I saw the double-bracket first. I search google, take some time, and then saw the TOML doc, recall that we can have more than one executable in Cargo, understood it. So it can be added to help someone like me (: