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>

tools/testing/selftests/tc-testing/README

@@ -128,7 +128,9 @@ optional arguments:
   -v, --verbose         Show the commands that are being run
   -N, --notap           Suppress tap results for command under test
   -d DEVICE, --device DEVICE
-                        Execute the test case in flower category
+                        Execute test cases that use a physical device, where
+                        DEVICE is its name. (If not defined, tests that require
+                        a physical device will be skipped)
   -P, --pause           Pause execution just before post-suite stage
 
 selection: