keys: Update comment for restrict_link_by_key_or_keyring_chain

Add the bit of information that makes
restrict_link_by_key_or_keyring_chain different from
restrict_link_by_key_or_keyring to the inline docs comment.

Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>