uio.9: Improve description of uio_rw flags

The direction of the data transfer in uio(9) can be confusing,
so state explicitly the difference between UIO_READ and UIO_WRITE.

Reviewed by: ziaee
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52996

(cherry picked from commit 0625f470e0956a3a337e4999f6712ec7b7d872e5)