Improve `Spi::new` documentation #1290

Job	Run time
Build rp2040-hal's examples	1m 26s
Verifiy build on MSRV	6m 32s
Build on-target-tests	58s
Build rp2040-hal's features	1m 32s
Execute host-runable tests	2m 41s
Check rp2040-hal for unused dependencies	2m 1s
	15m 10s

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