doc: clarify endianness assumption in readsb

[kernelpoetlaureate]

The readsb() function directly copies the on-disk superblock into the
in-memory struct. This is safe because the mkfs tool writes the filesystem
image in little-endian format, and the RISC-V kernel also runs on a
little-endian architecture.

Add a comment to make this implicit design assumption explicit. This
clarifies for students why direct memory copying works here and highlights a
key consideration for system portability.