include: spi: Clarify data frame units and meaning

Make clear in the include/ header that data frame size
is the same thing as word size for the context of this API.

Also, add some comments to the spi_context to make it easier
for driver writers to understand how to use the functions,
by noting the meaning of the dfs and len parameters to the update
functions. Otherwise it takes some time to understand what they mean.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>