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>
diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst
index 0733565..415be8e 100644
--- a/Documentation/security/index.rst
+++ b/Documentation/security/index.rst
@@ -5,5 +5,6 @@
 .. toctree::
    :maxdepth: 1
 
+   credentials
    IMA-templates
    tpm/index