doc: ReSTify credentials.txt

This updates the credentials API documentation to ReST markup and moves
it under the security subsection of kernel API documentation.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/security/index.rst

@@ -5,5 +5,6 @@ Security Documentation
 .. toctree::
    :maxdepth: 1
 
+   credentials
    IMA-templates
    tpm/index