doc: Bluetooth: Mesh: Clarify AES usage in DFU metadata

Added clarification on AES-CMAC usage in DFU metadata, emphasizing its
sole role as a hashing function for the incoming composition data hash.
Notably, it utilizes a fixed key and is not employed for encryption
purposes.

Signed-off-by: Stine Åkredalen <stine.akredalen@nordicsemi.no>