[docs] Add wpiunits to JavaDocs (#5793)

Co-authored-by: Sam Carlberg <sam.carlberg@gmail.com>

wpiunits/src/main/java/edu/wpi/first/units/UnaryFunction.java

@@ -8,6 +8,12 @@ import java.util.Objects;
 
 @FunctionalInterface
 public interface UnaryFunction {
+  /**
+   * Applies this function to the input value and returns the result.
+   *
+   * @param input the input value to the function
+   * @return the result
+   */
   double apply(double input);
 
   /**
@@ -15,13 +21,14 @@ public interface UnaryFunction {
    * input.
    *
    * <pre>
-   * f = x -> x + 1 // f(x) = x + 1
-   * g = x -> 2 * x // g(x) = 2x
+   * f = x -&gt; x + 1 // f(x) = x + 1
+   * g = x -&gt; 2 * x // g(x) = 2x
    *
    * h = f.pipeTo(g) // h(x) = g(f(x))
    * </pre>
    *
    * @param next the next operation to pipe to
+   * @return the composite function g(f(x))
    */
   default UnaryFunction pipeTo(UnaryFunction next) {
     Objects.requireNonNull(next, "The next operation in the chain must be provided");
@@ -29,19 +36,34 @@ public interface UnaryFunction {
     return x -> next.apply(this.apply(x));
   }
 
-  /** h(x) = f(x) * g(x). */
+  /**
+   * Creates a composite function h(x) such that h(x) = f(x) * g(x).
+   *
+   * @param multiplier the function to multiply this one by
+   * @return the composite function f(x) * g(x)
+   */
   default UnaryFunction mult(UnaryFunction multiplier) {
     Objects.requireNonNull(multiplier, "A multiplier function must be provided");
 
     return x -> this.apply(x) * multiplier.apply(x);
   }
 
-  /** g(x) = f(x) * k. */
+  /**
+   * Creates a composite function h(x) such that h(x) = k * f(x).
+   *
+   * @param multiplier the constant value to multiply this function's results by
+   * @return the composite function k * f(x)
+   */
   default UnaryFunction mult(double multiplier) {
     return x -> this.apply(x) * multiplier;
   }
 
-  /** h(x) = f(x) / g(x). */
+  /**
+   * Creates a composite function h(x) such that h(x) = f(x) / g(x).
+   *
+   * @param divisor the function to divide this one by
+   * @return the composite function f(x) / g(x)
+   */
   default UnaryFunction div(UnaryFunction divisor) {
     Objects.requireNonNull(divisor, "A divisor function must be provided");
 
@@ -59,18 +81,34 @@ public interface UnaryFunction {
     };
   }
 
-  /** g(x) * f(x) / k. */
+  /**
+   * Creates a composite function h(x) such that h(x) = 1/k * f(x).
+   *
+   * @param divisor the constant value to divide this function's results by
+   * @return the composite function 1/k * f(x)
+   */
   default UnaryFunction div(double divisor) {
     return x -> this.apply(x) / divisor;
   }
 
-  /** h(x) = f(x) ^ g(x). */
+  /**
+   * Creates a composite function h(x) such that h(x) = f(x) ^ g(x).
+   *
+   * @param exponent the function to exponentiate this function's results by
+   * @return the composite function f(x) ^ g(x)
+   */
   default UnaryFunction exp(UnaryFunction exponent) {
     Objects.requireNonNull(exponent, "An exponent function must be provided");
 
     return x -> Math.pow(this.apply(x), exponent.apply(x));
   }
 
+  /**
+   * Creates a composite function h(x) such that h(x) = f(x) ^ k.
+   *
+   * @param exponent the constant value to exponentiate this function's results by
+   * @return the composite function f(x) ^ k
+   */
   default UnaryFunction exp(double exponent) {
     return x -> Math.pow(this.apply(x), exponent);
   }