config: Update documentation for XTS option

The XTS configuration option MBEDTLS_CIPHER_MODE_XTS currently only enables
XTS for AES. So, don't say it enables XTS for "symmetric ciphers", just
AES. This helps to avoid being misleading.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index c7cf57a..e90893f 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -504,8 +504,7 @@
 /**
  * \def MBEDTLS_CIPHER_MODE_XTS
  *
- * Enable Xor-encrypt-xor with ciphertext stealing mode (XTS) for symmetric
- * ciphers.
+ * Enable Xor-encrypt-xor with ciphertext stealing mode (XTS) for AES.
  */
 #define MBEDTLS_CIPHER_MODE_XTS