spi: docs: update info about 'delay_usecs'

The 'delay_usecs' field is no longer present on the spi_transfer struct.
This change updates the doc to mention the usage of the (relatively) new
'delay' field.
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210308145502.1075689-11-aardelean@deviqon.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/spi/spi-summary.rst

@@ -411,8 +411,11 @@ any more such messages.
         duplex (one pointer is NULL) transfers;
 
       + optionally defining short delays after transfers ... using
-        the spi_transfer.delay_usecs setting (this delay can be the
-        only protocol effect, if the buffer length is zero);
+        the spi_transfer.delay.value setting (this delay can be the
+        only protocol effect, if the buffer length is zero) ...
+        when specifying this delay the default spi_transfer.delay.unit
+        is microseconds, however this can be adjusted to clock cycles
+        or nanoseconds if needed;
 
       + whether the chipselect becomes inactive after a transfer and
         any delay ... by using the spi_transfer.cs_change flag;