ui/paint: expand documentation and add package description

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/paint/doc.go

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+/*
+Package paint provides operations for 2D graphics.
+
+The PaintOp operation draws the current material into a rectangular
+area, taking the current clip path and transformation into account.
+
+The material is set by either a ColorOp for a constant color, or
+ImageOp for an image.
+
+The ClipOp operation sets the clip path. Drawing outside the clip
+path is ignored. A path is a closed shape of lines or curves.
+*/
+package paint