[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/PrintCommand.h

@@ -12,6 +12,8 @@
 namespace frc2 {
 /**
  * A command that prints a string when initialized.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class PrintCommand : public CommandHelper<InstantCommand, PrintCommand> {
  public: