[docs] Build with JavaDoc 17 and add missing docs (#6220)

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

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

@@ -6,6 +6,13 @@ package edu.wpi.first.units;
 
 import java.util.Objects;
 
+/**
+ * A function that accepts a single {@code double} and returns a {@code double} result. This is used
+ * to represent arbitrary mapping functions for converting units to and from a base unit
+ * representation. Temperature units, in particular, typically have an offset from a value in Kelvin
+ * and may have a multiplication factor added in, which means that units cannot always be
+ * represented as simple ratios of their base units.
+ */
 @FunctionalInterface
 public interface UnaryFunction {
   /**