[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/FunctionalCommand.h

@@ -19,6 +19,8 @@ namespace frc2 {
  * complex commands - note, however, that if a command is beyond a certain
  * complexity it is usually better practice to write a proper class for it than
  * to inline it.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class FunctionalCommand : public CommandHelper<CommandBase, FunctionalCommand> {
  public: