lib/base64: fix copy-pasted @padding doc in base64_decode()

lib/base64: fix copy-pasted @padding doc in base64_decode()

The @padding kernel-doc for base64_decode() says "whether to append '='
padding characters", which was copy-pasted from base64_encode().  In the
decode context, it controls whether the input is expected to include
padding, not whether to append it.

Link: https://lore.kernel.org/20260324223210.47676-3-objecting@objecting.org
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>