[generator] Merge Javadoc return values on bound properties (#995)

Context: 0e01fb5
Context: https://developer.android.com/reference/android/animation/Keyframe#setInterpolator(android.animation.TimeInterpolator)

The Java source for `Keyframe.setInterpolator` incidentally includes
a `@return` tag, even though the return type is void:

	/**
	 * Sets the optional interpolator for this Keyframe. A value of <code>null</code> indicates
	 * that there is no interpolation, which is the same as linear interpolation.
	 *
	 * @return The optional interpolator for this Keyframe.
	 */
	public void setInterpolator(TimeInterpolator interpolator) {
	    mInterpolator = interpolator;
	}

This causes us to generate multiple `<returns>` C# doc comment tags
on the `Android.Animation.Keyframe.Interpolator` property, as the
setter and getter both include the same `@return` tag:

	/// <summary>Gets the optional interpolator for this Keyframe. -or- Sets the optional interpolator for this Keyframe.</summary>
	/// ...
	/// <returns>The optional interpolator for this Keyframe.</returns>
	/// <returns>The optional interpolator for this Keyframe.</returns>
	public virtual unsafe Android.Animation.ITimeInterpolator? Interpolator {

The `mdoc` tool does not handle this well, and will continuously
append a `<value/>` element on this property every time the tool runs
against our improperly formatted C# doc comments:

	<Docs>
	  <summary>Gets the optional interpolator for this Keyframe. -or- Sets the optional interpolator for this Keyframe.</summary>
	  <value>The optional interpolator for this Keyframe.</value>
	  <value>The optional interpolator for this Keyframe.</value>
	  <value>The optional interpolator for this Keyframe.</value>
	  …
	  <since version="Added in API level 11" />
	</Docs>

Fix this by attempting to merge `<returns/>` elements for property
getters and setters.

Author: pjcollins

tools/generator/SourceWriters/BoundProperty.cs

@@ -179,6 +179,7 @@ void AddJavadocs (Property property)
 
 				MergeSummary (memberDocs, setterDocs);
 				MergeRemarks (memberDocs, setterDocs);
+				MergeReturns (memberDocs, setterDocs);
 
 				memberDocs.Add (setterDocs.Nodes ());
 			}
@@ -223,5 +224,20 @@ static void MergeRemarks (XElement mergeInto, XElement mergeFrom)
 				toContent.Add (fromContent.Nodes ());
 			}
 		}
+
+		static void MergeReturns (XElement mergeInto, XElement mergeFrom)
+		{
+			var toContent = mergeInto.Element ("returns");
+			var fromContent = mergeFrom.Element ("returns");
+
+			if (toContent != null && fromContent != null) {
+				if (toContent.Value == fromContent.Value) {
+					fromContent.Remove ();
+				} else {
+					toContent.Add (" ");
+					toContent.Add (fromContent.Nodes ());
+				}
+			}
+		}
 	}
 }