Documentation fixes
Fix spacing, comments, and links in doxygen documentation.
diff --git a/include/mbedtls/pk_internal.h b/include/mbedtls/pk_internal.h
index 0a33087..42d6bc0 100644
--- a/include/mbedtls/pk_internal.h
+++ b/include/mbedtls/pk_internal.h
@@ -1,11 +1,16 @@
/**
* \file pk_internal.h
*
- * \brief Public Key cryptography abstraction layer: built-in key types
+ * \brief Public Key cryptography abstraction layer: internal definitions
*
- * This file contains built-in types for handling various key types using
- * the interface defined in pk_info.h.
+ * This file contains built-in types for handling natively supported key types
+ * using the interface defined in pk_info.h.
*
+ * \warning This file contains internal definitions for the library.
+ * The interfaces in this file may change in future versions of the
+ * library without notice.
+ */
+/*
* Copyright (C) 2006-2018, ARM Limited, All Rights Reserved
* SPDX-License-Identifier: Apache-2.0
*