vmcore-dmesg/man page: Update the vmcore-dmesg man page

The vmcore-dmesg utility has been in usage for several years,
and is pretty stable now.

So its useful now to modify its man page to indicate the same.
Also fix some minor formatting issues.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>