[docs] Fix wpilibj JavaDoc warnings (#6154)

wpilibc/src/main/native/include/frc/smartdashboard/SendableBuilderImpl.h

@@ -21,6 +21,9 @@
 
 namespace frc {
 
+/**
+ * Implementation detail for SendableBuilder.
+ */
 class SendableBuilderImpl : public nt::NTSendableBuilder {
  public:
   SendableBuilderImpl() = default;