docs/spi: add information for data size

[toolmacher]

Hi,
think that was forgotten: data size description of bits in the class SPI. I added the information that the data size can also be 16.

[pfalcon]

I would find "and is the data size" part a bit confusing, why I don't merge this and leave for further review/discussion.

[toolmacher]

how about:
"bits can be 8 or 16, stands for the data bytes"
or
" bits can be 8 or 16, and is the word size"

[johnlr]

I think it usually called the word size

bits word size - can be 8 or 16.

[dpgeorge]

Which port are you using that supports 16 bits wide SPI words?

[toolmacher]

I use v1.8.1.
https://github.com/micropython/micropython/blob/master/stmhal/spi.c#L439
I measured it with the oscilloscope and it works.
>>> from pyb import SPI
>>> spi = SPI(1,SPI.MASTER,baudrate=1000,polarity=1,phase=0,bits=16)
>>> spi.send_recv(5)

[dpgeorge]

I see, then the stmhal code works by accident, that's nice to know!

Regarding the docs, how about: "can be 8 or 16, and is the number of bits in each transferred word"

[pfalcon]

Applied with @dpgeorge's wording.