[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());
 }
 }
+
+staticvoidMergeReturns(XElementmergeInto,XElementmergeFrom)
+{
+vartoContent=mergeInto.Element("returns");
+varfromContent=mergeFrom.Element("returns");
+
+if(toContent!=null&&fromContent!=null){
+if(toContent.Value==fromContent.Value){
+fromContent.Remove();
+}else{
+toContent.Add(" ");
+toContent.Add(fromContent.Nodes());
+}
+}
+}
 }
 }