util/format: document NONE swizzle

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26192>

src/util/format/u_formats.h

@@ -667,7 +667,10 @@ enum pipe_swizzle {
    PIPE_SWIZZLE_W,
    PIPE_SWIZZLE_0,
    PIPE_SWIZZLE_1,
+
+   /* Non-existent format channel, not used for swizzle operations. */
    PIPE_SWIZZLE_NONE,
+
    PIPE_SWIZZLE_MAX, /**< Number of enums counter (must be last) */
 };