[commands] Update Command documentation (NFC) (#3881)

Add reference to which VendorDep the class is included in.
Add missing OldCommands C++ Documentation (copied from Java).

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

@@ -18,6 +18,8 @@
 namespace frc2 {
 /**
  * A Sendable base class for Commands.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class CommandBase : public Command,
                     public wpi::Sendable,