docs(sdcardio): describe the real shared-SPI CS invariant

The previous ".. important::" note said SD card init must come first on
a shared SPI bus. That rule happened to work when every co-resident CS
line had a hardware pull-up, but it is not the real invariant: what
actually matters is that every CS on the shared bus is in a known HIGH
(deselected) state before any SPI transaction. On boards where a CS
floats (e.g. the Feather RP2040 RFM, whose RFM_CS has no pull-up), it
must be driven HIGH in software; init order is secondary.