docs: minor improvements

README.md

@@ -32,7 +32,7 @@ Java 17 or newer. No additional runtime dependencies.
 
 ### Code
 
-The API is intentionally "flat", comprising a handful of static methods and simple record classes.
+The API is intentionally "flat", comprising a handful of static methods and simple records as results.
 To get refraction-corrected topocentric coordinates:
 
 ```java
@@ -59,14 +59,21 @@ public class App {
 }
 ```
 
-The SPA class includes a method to calculate the times of sunrise, sun transit, and sunset in one fell swoop:
+The SPA class includes methods to calculate the times of sunrise, sun transit, and sunset in one fell swoop. The actual 
+return type depends on the type of day (regular day, polar day, polar night).
 
 ```java
 var result=SPA.calculateSunriseTransitSet(
         dateTime,
         70.978, // latitude  
         25.974, // longitude
         69); // delta T
+
+if(result instanceof SunriseResult.RegularDay regular) {
+    System.out.println(regular);
+} else {
+    System.out.println("no sunrise or sunset today!");    
+}
 ```
 
 Twilight start and end times can be obtained like sunrise and sunset, but assuming a different horizon:

src/main/java/net/e175/klaus/solarpositioning/SPA.java

@@ -211,6 +211,7 @@ private record AlphaDelta(double alpha, double delta) {}
    *     and Terrestrial Time), in seconds. See {@link JulianDate#JulianDate(ZonedDateTime, double)}
    *     and {@link DeltaT}.
    * @throws IllegalArgumentException for nonsensical latitude/longitude
+   * @return An implementation of {@link SunriseResult} depending on the type of day.
    */
   public static SunriseResult calculateSunriseTransitSet(
       final ZonedDateTime day, final double latitude, final double longitude, final double deltaT) {
@@ -234,6 +235,7 @@ private record RiseSetParams(double nuDegrees, AlphaDelta[] alphaDeltas, double[
    * @param horizon Horizon (basically, elevation angle) to use as the sunrise/sunset definition.
    *     This can be used to calculate twilight times.
    * @throws IllegalArgumentException for nonsensical latitude/longitude
+   * @return An implementation of {@link SunriseResult} depending on the type of day.
    */
   public static SunriseResult calculateSunriseTransitSet(
       final ZonedDateTime day,
@@ -268,8 +270,8 @@ public static SunriseResult calculateSunriseTransitSet(
    *     and {@link DeltaT}.
    * @param horizons Horizons (basically, elevation angles) to use as the sunrise/sunset definition.
    *     This can be used to calculate twilight times.
-   * @return A Map with one key-value pair for each unique horizon type and result. This map may or
-   *     may not be mutable.
+   * @return A Map with one key-value pair for each unique horizon type and {@link SunriseResult}.
+   *     This map may or may not be mutable.
    * @throws IllegalArgumentException for nonsensical latitude/longitude
    */
   public static Map<Horizon, SunriseResult> calculateSunriseTransitSet(