lib/crc: crc32: Document crc32_le(), crc32_be(), and crc32c()

lib/crc: crc32: Document crc32_le(), crc32_be(), and crc32c()

Document these widely used functions.

Update kernel-api.rst to point to the correct place, instead of to
crc32-main.c which no longer contains kerneldoc comments.

Simplify the documentation in crc32poly.h to just point to the
corresponding functions, now that they are properly documented.  Change
the value of CRC32C_POLY_LE to lower case, for consistency.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250619183414.100082-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>