Update the PdfDocument docs with unsupported operations.

The PDF generating canvas we use does not yet support all
drawing operations. This change adds docs which operations
are not yet supported.

Change-Id: I8d5ee8c1e2a473ba783b5e62f23ab90a54bbf106
diff --git a/graphics/java/android/graphics/pdf/PdfDocument.java b/graphics/java/android/graphics/pdf/PdfDocument.java
index 066ae2b..81e523d 100644
--- a/graphics/java/android/graphics/pdf/PdfDocument.java
+++ b/graphics/java/android/graphics/pdf/PdfDocument.java
@@ -391,6 +391,31 @@
         /**
          * Gets the {@link Canvas} of the page.
          *
+         * <p>
+         * <strong>Note: </strong> There are some draw operations that are
+         * not yet supported by the canvas returned by this method. More
+         * specifically:
+         * <ul>
+         * <li>{@link Canvas#clipPath(android.graphics.Path)
+         *     Canvas.clipPath(android.graphics.Path)}</li>
+         * <li>All flavors of {@link Canvas#drawText(String, float, float,
+         *     android.graphics.Paint) Canvas.drawText(String, float, float,
+         *     android.graphics.Paint)}</li>
+         * <li>All flavors of {@link Canvas#drawPosText(String, float[],
+         *     android.graphics.Paint) Canvas.drawPosText(String, float[],
+         *     android.graphics.Paint)}</li>
+         * <li>{@link Canvas#drawVertices(android.graphics.Canvas.VertexMode, int,
+         *     float[], int, float[], int, int[], int, short[], int, int,
+         *     android.graphics.Paint) Canvas.drawVertices(
+         *     android.graphics.Canvas.VertexMode, int, float[], int, float[],
+         *     int, int[], int, short[], int, int, android.graphics.Paint)}</li>
+         * <li>{@link android.graphics.PorterDuff.Mode#SRC_ATOP PorterDuff.Mode SRC},
+         *     {@link android.graphics.PorterDuff.Mode#DST_ATOP PorterDuff.DST_ATOP},
+         *     {@link android.graphics.PorterDuff.Mode#XOR PorterDuff.XOR},
+         *     {@link android.graphics.PorterDuff.Mode#ADD PorterDuff.ADD}</li>
+         * <li>Perspective transforms</li>
+         * </ul>
+         *
          * @return The canvas if the page is not finished, null otherwise.
          *
          * @see PdfDocument#finishPage(Page)