[cmd] Fix WaitUntilCommand for match time counting down (#8632)

Fixes #8631
Documents that it will return immediately if FMS isn't attached or DS
isn't in practice mode.
Related to change in DS match time behavior that was documented in #8606

wpilibNewCommands/src/main/native/include/frc2/command/WaitUntilCommand.h

@@ -36,7 +36,12 @@ class WaitUntilCommand : public CommandHelper<Command, WaitUntilCommand> {
    * will be judged to be legal by the referees.  When in doubt, add a safety
    * factor or time the action manually.
    *
+   * The match time counts down when connected to FMS or the DS is in practice
+   * mode for the current mode. When the DS is not connected to FMS or in
+   * practice mode, the command will not wait.
+   *
    * @param time the match time after which to end, in seconds
+   * @see frc::DriverStation::GetMatchTime()
    */
   explicit WaitUntilCommand(units::second_t time);