avutil/rc4: Add doxy for missing arguments

libavutil/rc4.h

@@ -42,6 +42,8 @@ AVRC4 *av_rc4_alloc(void);
 /**
  * @brief Initializes an AVRC4 context.
  *
+ * @param d pointer to the AVRC4 context
+ * @param key buffer containig the key
  * @param key_bits must be a multiple of 8
  * @param decrypt 0 for encryption, 1 for decryption, currently has no effect
  * @return zero on success, negative value otherwise
@@ -51,6 +53,7 @@ int av_rc4_init(struct AVRC4 *d, const uint8_t *key, int key_bits, int decrypt);
 /**
  * @brief Encrypts / decrypts using the RC4 algorithm.
  *
+ * @param d pointer to the AVRC4 context
  * @param count number of bytes
  * @param dst destination array, can be equal to src
  * @param src source array, can be equal to dst, may be NULL