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

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.

Open the linked selection algorithm details page and locate the “Unconditionally-refined tiles” section. Update the explanation of the geometric-error case to state that it applies only when the tile has no content, then review the rendered documentation to confirm the nuance is clear.

Written by the indexing model from the issue text.