#649 Link to web doc for background props from source

This might work if we maintain it.

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/parser/property/BackgroundPropertyBuilder.java

@@ -31,7 +31,10 @@
 import com.openhtmltopdf.css.parser.FSRGBColor;
 import com.openhtmltopdf.css.parser.PropertyValue;
 import com.openhtmltopdf.css.sheet.PropertyDeclaration;
+import com.openhtmltopdf.util.WebDoc;
+import com.openhtmltopdf.util.WebDocLocations;
 
+@WebDoc(WebDocLocations.CSS_BACKGROUND_PROPERTIES)
 public class BackgroundPropertyBuilder extends AbstractPropertyBuilder {
     // [<'background-color'> || <'background-image'> || <'background-repeat'> || 
     // <'background-attachment'> || <'background-position'>] | inherit 

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/parser/property/PrimitiveBackgroundPropertyBuilders.java

@@ -16,7 +16,10 @@
 import com.openhtmltopdf.css.parser.Token;
 import com.openhtmltopdf.css.parser.property.PrimitivePropertyBuilders.GenericColor;
 import com.openhtmltopdf.css.sheet.PropertyDeclaration;
+import com.openhtmltopdf.util.WebDoc;
+import com.openhtmltopdf.util.WebDocLocations;
 
+@WebDoc(WebDocLocations.CSS_BACKGROUND_PROPERTIES)
 public class PrimitiveBackgroundPropertyBuilders {
     private static BitSet setOf(IdentValue... val) {
         return PrimitivePropertyBuilders.setFor(val);

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/style/CalculatedStyle.java

@@ -21,6 +21,7 @@
 package com.openhtmltopdf.css.style;
 
 import java.awt.Cursor;
+import java.lang.annotation.Documented;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -55,6 +56,8 @@
 import com.openhtmltopdf.render.FSFontMetrics;
 import com.openhtmltopdf.render.RenderingContext;
 import com.openhtmltopdf.util.LogMessageId;
+import com.openhtmltopdf.util.WebDoc;
+import com.openhtmltopdf.util.WebDocLocations;
 import com.openhtmltopdf.util.XRLog;
 import com.openhtmltopdf.util.XRRuntimeException;
 
@@ -1388,6 +1391,11 @@ public FSLinearGradient getLinearGradient(
         return new FSLinearGradient(this, value.getFunction(), boxWidth, boxHeight, cssContext);
     }
 
+    /**
+     * Gets the values of the background properties and combines in a list
+     * of BackgroundContainer values.
+     */
+    @WebDoc(WebDocLocations.CSS_BACKGROUND_PROPERTIES)
     public List<BackgroundContainer> getBackgroundImages() {
         List<PropertyValue> images = ((ListValue) valueByName(CSSName.BACKGROUND_IMAGE)).getValues();
         List<PropertyValue> positions = ((ListValue) valueByName(CSSName.BACKGROUND_POSITION)).getValues();

openhtmltopdf-core/src/main/java/com/openhtmltopdf/util/WebDoc.java

@@ -0,0 +1,25 @@
+package com.openhtmltopdf.util;
+
+import static java.lang.annotation.ElementType.CONSTRUCTOR;
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.SOURCE;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+
+/**
+ * Lets us specify a url where the type, method, etc
+ * is documented. Specified as an annotation so we can use
+ * IDE features to find instances and maybe tooling can use it too.
+ */
+@Retention(SOURCE)
+@Target({ TYPE, FIELD, METHOD, CONSTRUCTOR })
+public @interface WebDoc {
+    /**
+     * A url where docs can be found.
+     */
+    public String value();
+}

openhtmltopdf-core/src/main/java/com/openhtmltopdf/util/WebDocLocations.java

@@ -0,0 +1,5 @@
+package com.openhtmltopdf.util;
+
+public class WebDocLocations {
+    public static final String CSS_BACKGROUND_PROPERTIES = "https://github.com/danfickle/openhtmltopdf/wiki/Big-CSS-reference#background-properties";
+}