doc: Move vhost tx retry info to separate section.

doc: Move vhost tx retry info to separate section.

vhost tx retry is applicable to vhost-user and vhost-user-client,
but was in the section that compares them. Also, moved further
down the doc as prefer to have more fundamental info about vhost
nearer the top.

Fixes: 6d6513bfc657 ("doc: Add info on vhost tx retries.")
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>