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>