drivers: spi: document expectation on spi config parameter

Most if not all drivers use an internal function
spi_context_configured() to bypass reconfiguring the SPI peripheral
when nothing has changed.  That function determines change based on
comparing the struct spi_config pointer that was last used.  This does
not work if a user changes fields within the pointed-to structure.

Document that pointer comparison may be used to detect changes.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>