dm dust: convert documentation to ReST

Convert the dm-dust documentation to ReST formatting, using literal
blocks for all of the shell command, shell output, and log output
examples.

Add dm-dust to index.rst.

Additionally, fix an annotation in the "querying for specific bad
blocks" section, on the "queryblock ... not found in badblocklist"
example, to properly state that the message appears when a given
block is not found.

Signed-off-by: Bryan Gurney <bgurney@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>