Doc: Explanation of unconditionally-refined tiles in selection algorithm doc is missing nuance about empty content

[kring]

This section in the doc is mention an important bit of information: https://cesium.com/learn/cesium-native/ref-doc/selection-algorithm-details.html#unconditionally-refined-tiles

It says:

Unconditionally-refined tiles are used in the following situations: ... 3. Tiles that have a geometric error that is higher than their parent's.

This is mostly true, but the other important requirement is that the tile not have any content. A tile that has renderable content will never be treated as unconditionally-refined, no matter its geometric error.