docs/library/pyb.SPI: init(): Describe "bits" argument.

Based on https://github.com/micropython/micropython/pull/2210 .

docs/library/pyb.SPI.rst

@@ -68,6 +68,7 @@ Methods
          - ``polarity`` can be 0 or 1, and is the level the idle clock line sits at.
          - ``phase`` can be 0 or 1 to sample data on the first or second clock edge
            respectively.
+         - ``bits`` can be 8 or 16, and is the number of bits in each transferred word.
          - ``firstbit`` can be ``SPI.MSB`` or ``SPI.LSB``.
          - ``crc`` can be None for no CRC, or a polynomial specifier.