Point to some documentation

Change-Id: Ia467db7fd2ebaf85600e5a0c9f11f1e576363177
Surely we want CGFloat and CGRect from the appropriate SDK header

Especially as the CGFloat typedef depends on whether compiling 32- or
64-bit code.

Change-Id: I2426aa6fb91a9a76367eb965d08b985ce69e4997