Fix documentation warnings generated by JavaDoc (NFC) (#3428)

Some C++ Doxygen comments were updated to reflect any wording changes.

See `rg "(@return|@param \w+) TODO" | less` for list of incomplete docs.

wpimath/src/main/java/edu/wpi/first/math/StateSpaceUtil.java

@@ -157,6 +157,7 @@ public final class StateSpaceUtil {
    * radians.
    *
    * @param pose A pose to convert to a vector.
+   * @return The given pose in as a 4x1 vector of x, y, cos(theta), and sin(theta).
    */
   public static Matrix<N4, N1> poseTo4dVector(Pose2d pose) {
     return VecBuilder.fill(