lib/crc: document all the CRC library kconfig options

Previous commits removed all the original CRC kconfig help text, since
it was oriented towards people configuring the kernel, and the options
are no longer user-selectable.  However, it's still useful for there to
be help text for kernel developers.  Add this.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250401221600.24878-7-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>