rendernode, snapshot: Slightly expand color matrix docs

rendernode, snapshot: Slightly expand color matrix docs

Based on reverse engineering the color node and contrary to my
expectations, the matrix/offset is expressed in, and applied to,
unpremultiplied colors. The colors are being explicitly
unpremultiplied, transformed according to the matrix/offset, and
premultiplied back (see color_matrix.glsl). The matrix is getting
transposed.

Also, copy the same blurb to the corresponding GtkSnapshot function.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>