Implement support of @note doxygen command

[sudden6]

This example snippet shows a potential use of @note:

/**
 * @brief Looks up a key from the cache or computes it if it didn't exist yet.
 * @param cache Cache to perform the lookup on
 * @param public_key Public key, used for the lookup and computation.
 *
 * @return The shared key of length CRYPTO_SHARED_KEY_SIZE, matching the public key and our secret key or nullptr on error.
 * @note The cache executes a housekeeping cycle, to remove timed out keys, for the active slot on every lookup.
 */
non_null()
const uint8_t* shared_key_cache_lookup(Shared_Key_Cache *cache, const uint8_t public_key[CRYPTO_PUBLIC_KEY_SIZE]);