[commands] Improve documentation of addRequirements (NFC) (#5103)

wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/CommandBase.java

@@ -26,7 +26,11 @@ public abstract class CommandBase implements Sendable, Command {
   }
 
   /**
-   * Adds the specified requirements to the command.
+   * Adds the specified subsystems to the requirements of the command. The scheduler will prevent
+   * two commands that require the same subsystem from being scheduled simultaneously.
+   *
+   * <p>Note that the scheduler determines the requirements of a command when it is scheduled, so
+   * this method should normally be called from the command's constructor.
    *
    * @param requirements the requirements to add
    */