Add notes about extra args for swarmed tests

Adds several notes about running swarmed tests with extra arguments in
docs/workflow/debugging-with-swarming.md.

Change-Id: I751e0237399ff96626ef699b9a66e598eb07ab5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1793282
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694854}

docs/workflow/debugging-with-swarming.md

@@ -128,6 +128,10 @@ other dimensions include:
 The [swarming bot list] allows you to see all the dimensions and the values they
 can take on.
 
+If you need to pass additional arguments to the test, simply add
+`-- $extra_args` to the end of the `swarming.py trigger` command line - anything
+after the `--` will be passed directly to the test.
+
 When you invoke `swarming.py trigger`, it will emit two pieces of information: a
 URL for the task it created, and a command you can run to collect the results of
 that task. For example:
@@ -155,6 +159,22 @@ $ tools/run-swarmed.py $outdir $target
 
 See the `--help` option of `run-swarmed.py` for more details about that script.
 
+## mb.py run
+
+Similar to `tools/run_swarmed.py`, `mb.py run` bundles much of the logic into a
+single command line. Unlike `tools/run_swarmed.py`, `mb.py run` allows the user
+to specify extra arguments to pass to the test, but has a messier command line.
+
+To use it, run:
+```
+$ tools/mb/mb.py run \
+    -s --no-default-dimensions \
+    -d pool $pool \
+    $criteria \
+    $outdir $target \
+    -- $extra_args
+```
+
 ## Other notes
 
 If you are looking at a Swarming task page, be sure to check the bottom of the