Documented CSD parsing expectations for SDIO lower-half

[arjav1528]

Fixes: #17248

Summary

Addressed the issue by documenting how SDIO lower-half drivers must provide R2 (CID/CSD) responses and by adding a small sanity check in the MMCSD upper-half. include/nuttx/sdio.h now explains the R2 format, states that the upper-half expects a 128‑bit CID/CSD payload in the standard layout, and notes that controllers which strip the CRC must shift the four 32‑bit words left by one byte (with bcm2711_sdio.c and imx9_usdhc.c as examples). mmcsd_decode_csd in drivers/mmcsd/mmcsd_sdio.c gains a DEBUGASSERT(csd[0] != 0); under CONFIG_DEBUG_FEATURES to catch obviously invalid or misaligned CSDs, and the SDIO/MMCSD docs now describe the SDIO lower-half call-flow and R2/CSD expectations and point implementers to this guidance.

Impact

• Runtime behavior: Only a debug‑only DEBUGASSERT in mmcsd_decode_csd is added; there is no behavior or performance change in non‑debug builds.
• API/ABI: No changes to SDIO or MMCSD public interfaces; existing correct SDIO lower‑halves continue to work unchanged.
• Documentation: SDIO docs now explicitly describe recv_r2/R2 handling (including CRC‑stripping controllers), and MMCSD docs direct SDIO lower‑half implementers there.
• Build / compatibility / security: No new Kconfig options or dependencies beyond existing Sphinx for docs; no security impact; compatibility for /dev/mmcsd* users is unchanged.

Testing

• make html in Documentation/ to verify the updated RST documentation builds successfully.
• No runtime or hardware tests were performed because no SDIO/MMC-capable target hardware is available in this environment; the new DEBUGASSERT behavior is validated by static inspection only.