[docs] Add missing JavaDocs (#6146)

wpimath/src/main/native/include/frc/controller/ArmFeedforward.h

@@ -33,10 +33,10 @@ class WPILIB_DLLEXPORT ArmFeedforward {
   /**
    * Creates a new ArmFeedforward with the specified gains.
    *
-   * @param kS   The static gain, in volts.
+   * @param kS The static gain, in volts.
    * @param kG The gravity gain, in volts.
-   * @param kV   The velocity gain, in volt seconds per radian.
-   * @param kA   The acceleration gain, in volt seconds² per radian.
+   * @param kV The velocity gain, in volt seconds per radian.
+   * @param kA The acceleration gain, in volt seconds² per radian.
    */
   constexpr ArmFeedforward(
       units::volt_t kS, units::volt_t kG, units::unit_t<kv_unit> kV,
@@ -175,9 +175,16 @@ class WPILIB_DLLEXPORT ArmFeedforward {
     return MaxAchievableAcceleration(-maxVoltage, angle, velocity);
   }
 
+  /// The static gain, in volts.
   const units::volt_t kS;
+
+  /// The gravity gain, in volts.
   const units::volt_t kG;
+
+  /// The velocity gain, in volt seconds per radian.
   const units::unit_t<kv_unit> kV;
+
+  /// The acceleration gain, in volt seconds² per radian.
   const units::unit_t<ka_unit> kA;
 };
 }  // namespace frc

wpimath/src/main/native/include/frc/controller/DifferentialDriveWheelVoltages.h

@@ -12,7 +12,10 @@ namespace frc {
  * Motor voltages for a differential drive.
  */
 struct DifferentialDriveWheelVoltages {
+  /// Left wheel voltage.
   units::volt_t left = 0_V;
+
+  /// Right wheel voltage.
   units::volt_t right = 0_V;
 };
 

wpimath/src/main/native/include/frc/controller/ElevatorFeedforward.h

@@ -182,9 +182,16 @@ class ElevatorFeedforward {
     return MaxAchievableAcceleration(-maxVoltage, velocity);
   }
 
+  /// The static gain.
   const units::volt_t kS;
+
+  /// The gravity gain.
   const units::volt_t kG;
+
+  /// The velocity gain.
   const units::unit_t<kv_unit> kV;
+
+  /// The acceleration gain.
   const units::unit_t<ka_unit> kA;
 };
 }  // namespace frc

wpimath/src/main/native/include/frc/controller/SimpleMotorFeedforward.h

@@ -160,8 +160,13 @@ class SimpleMotorFeedforward {
     return MaxAchievableAcceleration(-maxVoltage, velocity);
   }
 
+  /** The static gain. */
   const units::volt_t kS;
+
+  /** The velocity gain. */
   const units::unit_t<kv_unit> kV;
+
+  /** The acceleration gain. */
   const units::unit_t<ka_unit> kA;
 };
 }  // namespace frc