Add some documentation concerning the cap.FuncLauncher() behavior.

Slavi Marinov was asking about how a single webserver might use the
cap package to serve different content as a different user? So I
realized this detail wasn't obvious from the package documentation.

I also put together this example sketch:

   https://play.golang.org/p/6Hr0XW3JP6a

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>
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