[GTK] Background images behavior differs from Windows and Mac

Document the intended behavior

Contributes to:
#2290

Author: N1k145

bundles/org.eclipse.swt/Eclipse SWT/cocoa/org/eclipse/swt/widgets/Control.java

@@ -3547,6 +3547,9 @@ void setBackground () {
  * <p>
  * Note: This operation is a hint and may be overridden by the platform.
  * </p>
+ * <p>
+ * Note: The background color can be overridden by setting a background image.
+ * </p>
  * @param color the new color (or null)
  *
  * @exception IllegalArgumentException <ul>
@@ -3587,6 +3590,9 @@ private void _setBackground (Color color) {
  * Note: This operation is a hint and may be overridden by the platform.
  * For example, on Windows the background of a Button cannot be changed.
  * </p>
+ * <p>
+ * Note: Setting a background image overrides a set background color.
+ * </p>
  * @param image the new image (or null)
  *
  * @exception IllegalArgumentException <ul>

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Control.java

@@ -5172,6 +5172,9 @@ void setBackground () {
  * <p>
  * Note: This operation is a hint and may be overridden by the platform.
  * </p>
+ * <p>
+ * Note: The background color can be overridden by setting a background image.
+ * </p>
  * @param color the new color (or null)
  *
  * @exception IllegalArgumentException <ul>
@@ -5277,6 +5280,9 @@ void setBackgroundGdkRGBA (long handle, GdkRGBA rgba) {
  * Note: This operation is a hint and may be overridden by the platform.
  * For example, on Windows the background of a Button cannot be changed.
  * </p>
+ * <p>
+ * Note: Setting a background image overrides a set background color.
+ * </p>
  * @param image the new image (or null)
  *
  * @exception IllegalArgumentException <ul>

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java

@@ -3072,6 +3072,9 @@ void setBackground () {
  * <p>
  * Note: This operation is a hint and may be overridden by the platform.
  * </p>
+ * <p>
+ * Note: The background color can be overridden by setting a background image.
+ * </p>
  * @param color the new color (or null)
  *
  * @exception IllegalArgumentException <ul>
@@ -3114,6 +3117,9 @@ private void _setBackground (Color color) {
  * Note: This operation is a hint and may be overridden by the platform.
  * For example, on Windows the background of a Button cannot be changed.
  * </p>
+ * <p>
+ * Note: Setting a background image overrides a set background color.
+ * </p>
  * @param image the new image (or null)
  *
  * @exception IllegalArgumentException <ul>