docs: Add remaining Documentation/*.txt at staging/index.rst

There are a few text files that aren't yet at the docs building
system.

Add those to the staging/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/35967d18de8b3ee480194acf68e62d515347c1e5.1592918949.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/staging/index.rst

@@ -19,17 +19,14 @@ Unsorted Documentation
 Atomic Types
 ============
 
-.. include:: ../atomic_t.txt
-   :literal:
+.. literalinclude:: ../atomic_t.txt
 
 Atomic bitops
 =============
 
-.. include:: ../atomic_bitops.txt
-   :literal:
+.. literalinclude:: ../atomic_bitops.txt
 
 Memory Barriers
 ===============
 
-.. include:: ../memory-barriers.txt
-   :literal:
+.. literalinclude:: ../memory-barriers.txt