ssl_cache: Error renaming and document improvement

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/include/mbedtls/ssl_cache.h b/include/mbedtls/ssl_cache.h
index 9b7d8e6..7009827 100644
--- a/include/mbedtls/ssl_cache.h
+++ b/include/mbedtls/ssl_cache.h
@@ -104,7 +104,8 @@
  *                        associated with \p session_id, if present.
  *
  * \return                \c 0 on success.
- * \return                A negative error code on failure.
+ * \return                #MBEDTLS_ERR_SSL_CACHE_ENTRY_NOT_FOUND if there is
+ *                        no cache entry with specified session ID found.
  */
 int mbedtls_ssl_cache_get(void *data,
                           unsigned char const *session_id,