Fix inconsistent documentation of cipher_setup()

- the \internal note said that calling cipher_init() first would be made
mandatory later, but the documention of the ctx parameter already said
the context had to be initialized...
- the documentation was using the word initialize for two different
meanings (calling setup() vs calling init()), making the documentation
of the ctx parameter quite confusing (you must initialize before you can
initialize...)

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

include/mbedtls/cipher.h

@@ -437,10 +437,11 @@ void mbedtls_cipher_free( mbedtls_cipher_context_t *ctx );
 
 
 /**
- * \brief               This function initializes a cipher context for
+ * \brief               This function prepares a cipher context for
  *                      use with the given cipher primitive.
  *
- * \param ctx           The context to initialize. This must be initialized.
+ * \param ctx           The context to prepare. This must be initialized by
+ *                      a call to mbedtls_cipher_init() first.
  * \param cipher_info   The cipher to use.
  *
  * \return              \c 0 on success.
@@ -448,10 +449,6 @@ void mbedtls_cipher_free( mbedtls_cipher_context_t *ctx );
  *                      parameter-verification failure.
  * \return              #MBEDTLS_ERR_CIPHER_ALLOC_FAILED if allocation of the
  *                      cipher-specific context fails.
- *
- * \internal Currently, the function also clears the structure.
- * In future versions, the caller will be required to call
- * mbedtls_cipher_init() on the structure first.
  */
 int mbedtls_cipher_setup( mbedtls_cipher_context_t *ctx,
                           const mbedtls_cipher_info_t *cipher_info );