Documentation: Add io_ordering.rst to driver-api manual

Add io_ordering.rst under Documentation/driver-api and reference it from
the Sphinx TOC Tree present in Documentation/driver-api/index.rst

Signed-off-by: Pragat Pandya <pragat.pandya@gmail.com>
Link: https://lore.kernel.org/r/20200303050301.5412-3-pragat.pandya@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index 99bdb39..d4e78cb 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -80,6 +80,7 @@
    isa
    isapnp
    io-mapping
+   io_ordering
    generic-counter
    lightnvm-pblk
    memory-devices/index
diff --git a/Documentation/io_ordering.txt b/Documentation/driver-api/io_ordering.rst
similarity index 100%
rename from Documentation/io_ordering.txt
rename to Documentation/driver-api/io_ordering.rst