[bazel/docs] Document test tags
[bazel/docs] Document test tags

Signed-off-by: Drew Macrae <32d48ac45113625c0a04afe7343eae478a303684@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan