docs/sdio: clarify 136-bit R2 format

This update adds comprehensive documentation regarding the 136-bit response format for SDIO, including the structure of the response, CRC handling, and necessary byte-shifting for alignment. This information is crucial for developers implementing or maintaining SDIO drivers.

Signed-off-by: Arjav Patel <arjav1528@gmail.com>

include/nuttx/sdio.h

@@ -693,6 +693,20 @@
  *   status.  The driver implementation should verify the correctness of
  *   the remaining, non-returned bits (CRCs, CMD index, etc.).
  *
+ *   SDIO_RECVR2 (136-bit response): The 136-bit response has format: start
+ *   bit (1), transmission bit (0), 6 reserved bits (all 1), 128 bits of
+ *   CID/CSD data, 7-bit CRC, and end bit (1). The upper-half expects the
+ *   buffer to contain exactly 128 bits of payload in the standard layout
+ *   (e.g., r2[0] contains bits 127-96, r2[1] bits 95-64, etc.).
+ *
+ *   The upper-half does not verify the CRC; that is the responsibility of
+ *   the lower-half. If the SDMMC controller hardware removes the trailing
+ *   CRC byte before storing the response, the resulting 128 bits will be
+ *   misaligned (often shifted right by 8 bits with leading zeros). In this
+ *   case, the lower-half implementation MUST shift the four 32-bit words
+ *   left by one byte to restore the expected 128-bit payload alignment.
+ *   See bcm2711_sdio.c or imx9_usdhc.c for examples of this byte-shifting.
+ *
  * Input Parameters:
  *   dev    - An instance of the SDIO device interface
  *   Rx - Buffer in which to receive the response