spi: documentation tweaks

Update SPI documentation to clarify some areas of recent confusion: clock
polarity takes effect when chipselect goes active; and zero length buffers are
OK in certain cases.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>