Update aes.h

Minor documentation improvements:
Standardized brief file description.
Split returns.
Minor fixes.
diff --git a/include/mbedtls/aes.h b/include/mbedtls/aes.h
index 46016dc..c82d39a 100644
--- a/include/mbedtls/aes.h
+++ b/include/mbedtls/aes.h
@@ -1,7 +1,9 @@
 /**
  * \file aes.h
  *
- * \brief   The Advanced Encryption Standard (AES) specifies a FIPS-approved
+ * \brief   This file contains AES definitions and functions.
+ *
+ *          The Advanced Encryption Standard (AES) specifies a FIPS-approved
  *          cryptographic algorithm that can be used to protect electronic
  *          data.
  *
@@ -12,6 +14,7 @@
  *          techniques -- Encryption algorithms -- Part 2: Asymmetric
  *          ciphers</em>.
  */
+
 /*  Copyright (C) 2006-2018, Arm Limited (or its affiliates), All Rights Reserved.
  *  SPDX-License-Identifier: Apache-2.0
  *
@@ -112,8 +115,8 @@
  *                 <li>192 bits</li>
  *                 <li>256 bits</li></ul>
  *
- * \return         \c 0 on success or #MBEDTLS_ERR_AES_INVALID_KEY_LENGTH
- *                 on failure.
+ * \return         \c 0 on success.
+ *                 #MBEDTLS_ERR_AES_INVALID_KEY_LENGTH on failure.
  */
 int mbedtls_aes_setkey_enc( mbedtls_aes_context *ctx, const unsigned char *key,
                     unsigned int keybits );
@@ -128,7 +131,8 @@
  *                 <li>192 bits</li>
  *                 <li>256 bits</li></ul>
  *
- * \return         \c 0 on success, or #MBEDTLS_ERR_AES_INVALID_KEY_LENGTH on failure.
+ * \return         \c 0 on success.
+ * \return         #MBEDTLS_ERR_AES_INVALID_KEY_LENGTH on failure.
  */
 int mbedtls_aes_setkey_dec( mbedtls_aes_context *ctx, const unsigned char *key,
                     unsigned int keybits );
@@ -192,7 +196,8 @@
  * \param input    The buffer holding the input data.
  * \param output   The buffer holding the output data.
  *
- * \return         \c 0 on success, or #MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH
+ * \return         \c 0 on success.
+ * \return         #MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH
  *                 on failure.
  */
 int mbedtls_aes_crypt_cbc( mbedtls_aes_context *ctx,
@@ -313,7 +318,7 @@
  * \param input            The buffer holding the input data.
  * \param output           The buffer holding the output data.
  *
- * \return     \c 0 on success.
+ * \return                 \c 0 on success.
  */
 int mbedtls_aes_crypt_ctr( mbedtls_aes_context *ctx,
                        size_t length,
@@ -406,7 +411,8 @@
 /**
  * \brief          Checkup routine.
  *
- * \return         \c 0 on success, or \c 1 on failure.
+ * \return         \c 0 on success.
+ * \return         \c 1 on failure.
  */
 int mbedtls_aes_self_test( int verbose );