tc-testing: Clarify the use of tdc's -d option

The -d command line argument to tdc requires the name of a physical device
on the system where the tests will be run. If -d has not been used, tdc
will skip tests that require a physical device.

This patch is intended to better document what the -d option does and how
it is used.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>