Add info about how iText works with opacity.

DEVSIX-1358

Author: IdamkinI

io/src/main/java/com/itextpdf/io/LogMessageConstant.java

@@ -53,7 +53,7 @@ public final class LogMessageConstant {
     public static final String ATTEMPT_TO_MOVE_TO_FLUSHED_PARENT = "An attempt is made to move the tag tree pointer to the tag parent which has been already flushed. Tag tree pointer is moved to the root tag instead.";
     public static final String CALCULATE_HASHCODE_FOR_MODIFIED_PDFNUMBER = "Calculate hashcode for modified PdfNumber.";
     public static final String CLIP_ELEMENT = "Element content was clipped because some height properties are set.";
-    public static final String COLOR_ALPHA_CHANNEL_IS_IGNORED = "Alpha channel {0} was ignored during creation of color";
+    public static final String COLOR_ALPHA_CHANNEL_IS_IGNORED = "Alpha channel {0} was ignored during color creation. Note that opacity can be achieved in some places by using 'setOpacity' method or 'TransparentColor' class";
     public static final String COLORANT_INTENSITIES_INVALID = "Some of colorant intensities are invalid: they are bigger than 1 or less than 0. We will force them to become 1 or 0 respectively.";
     public static final String COLOR_NOT_FOUND = "Color \"{0}\" not found.";
     public static final String COLOR_NOT_PARSED = "Color \"{0}\" was not parsed. It has invalid value. Defaulting to black color.";

kernel/src/main/java/com/itextpdf/kernel/color/DeviceRgb.java

@@ -118,7 +118,9 @@ public DeviceRgb(float r, float g, float b) {
 
     /**
      * Create DeviceRGB color from R, G, B values of java.awt.Color
-     * Note, that alpha chanel is ignored
+     * <br/>
+     * Note, that alpha chanel is ignored,  but opacity still can be achieved
+     * in some places by using 'setOpacity' method or 'TransparentColor' class.
      *
      * @param color the color which RGB values are used
      */