Doc: Explain ImageParticle lookup tables

The documentation for the size and opacity lookup tables on
ImageParticle is one sentence each. That sentence doesn't say how
the image maps onto a particle's lifetime, or which channel matters.
The opacity table's description is copied from the size table's, so
it says the image controls size. Both entries also say the property
is about to be removed in favor of easing curves. That hasn't
happened, and the warning puts readers off a working feature.

The entries explain that the image is read from left to right over
the particle's life, and that only the alpha channel counts. They
also give the sampling resolution, so authors know a narrow gradient
image is enough. Each entry says how its table combines with the
emitter's size settings or with the particle's alpha and entry
effect, and links to the related properties. The removal warning is
gone, so readers can use the tables with confidence.

Pick-to: 6.12
Fixes: QTBUG-50249
Change-Id: I0769663a803eab5b00a7d377ead809c741b7baba
Reviewed-by: Topi Reinio <topi.reinio@qt.io>
Reviewed-by: David Boddie <david.boddie@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>