[wpiunits] Documentation improvements (#5932)

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

@@ -4,6 +4,15 @@
 
 package edu.wpi.first.units;
 
+/**
+ * Unit of angular dimension.
+ *
+ * <p>This is the base type for units of distance dimension. It is also used to specify the
+ * dimension for {@link Measure}: <code>Measure&lt;Distance&gt;</code>.
+ *
+ * <p>Actual units (such as {@link Units#Meters} and {@link Units#Inches}) can be found in the
+ * {@link Units} class.
+ */
 public class Distance extends Unit<Distance> {
   /** Creates a new unit with the given name and multiplier to the base unit. */
   Distance(double baseUnitEquivalent, String name, String symbol) {