docs: crypto: convert async-tx-api.txt to ReST format

- Place the txt index inside a comment;
- Use title and chapter markups;
- Adjust markups for numbered list;
- Mark literal blocks as such;
- Use tables markup.
- Adjust indentation when needed.

Acked-By: Vinod Koul <vkoul@kernel.org> # dmaengine
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/98977242130efe86d1200f7a167299d4c1c205c5.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dmaengine/client.rst

@@ -5,7 +5,7 @@ DMA Engine API Guide
 Vinod Koul <vinod dot koul at intel.com>
 
 .. note:: For DMA Engine usage in async_tx please see:
-          ``Documentation/crypto/async-tx-api.txt``
+          ``Documentation/crypto/async-tx-api.rst``
 
 
 Below is a guide to device driver writers on how to use the Slave-DMA API of the

Documentation/driver-api/dmaengine/provider.rst

@@ -95,7 +95,7 @@ accommodates that API in some cases, and made some design choices to
 ensure that it stayed compatible.
 
 For more information on the Async TX API, please look the relevant
-documentation file in Documentation/crypto/async-tx-api.txt.
+documentation file in Documentation/crypto/async-tx-api.rst.
 
 DMAEngine APIs
 ==============