[wpiunits] Documentation improvements (#5932)

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

@@ -4,6 +4,15 @@
 
 package edu.wpi.first.units;
 
+/**
+ * Unit of energy dimension.
+ *
+ * <p>This is the base type for units of energy dimension. It is also used to specify the dimension
+ * for {@link Measure}: <code>Measure&lt;Energy&gt;</code>.
+ *
+ * <p>Actual units (such as {@link Units#Joules} and {@link Units#Kilojoules}) can be found in the
+ * {@link Units} class.
+ */
 public class Energy extends Unit<Energy> {
   protected Energy(
       UnaryFunction toBaseConverter, UnaryFunction fromBaseConverter, String name, String symbol) {