doc: ReSTify seccomp_filter.txt

This updates seccomp_filter.txt for ReST markup, and moves it under the
user-space API index, since it describes how application author can use
seccomp.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/userspace-api/index.rst

@@ -16,6 +16,7 @@ place where this information is gathered.
 .. toctree::
    :maxdepth: 2
 
+   seccomp_filter
    unshare
 
 .. only::  subproject and html

MAINTAINERS

@@ -11492,6 +11492,7 @@ F:	kernel/seccomp.c
 F:	include/uapi/linux/seccomp.h
 F:	include/linux/seccomp.h
 F:	tools/testing/selftests/seccomp/*
+F:	Documentation/userspace-api/seccomp_filter.rst
 K:	\bsecure_computing
 K:	\bTIF_SECCOMP\b