Missing documentation for `spi.read()` second argument

[microbit-carlos]

Present in V1 and V2, it essentially writes to the SPI out the value of the second argument, for as long as it is reading data.

V1: https://github.com/bbcmicrobit/micropython/blob/v1.1.1/source/microbit/microbitspi.cpp#L111-L117

V2:

micropython-microbit-v2/src/codal_port/microbit_spi.c

Lines 109 to 114 in 15b467c

	uint8_t byte_out = 0;
	if (n_args == 3) {
	byte_out = mp_obj_get_int(args[2]);
	}
	memset(vstr.buf, byte_out, vstr.len);
	int ret = microbit_hal_spi_transfer(vstr.len, (uint8_t *)vstr.buf, (uint8_t *)vstr.buf);

[microbit-carlos]

Docs PR:

• docs: Add spi.read() second argument. bbcmicrobit/micropython#805

[microbit-carlos]

Docs PR merged.