Confusingly, these two properties had identical descriptions even though they measure different things.
"relative_index" measures character count from the custom effect's bbcode opening tag.
"absolute_index" measures character count from the start of the bbcode text that includes the custom effect.
See the code author's own explanation here: https://github.com/godotengine/godot/pull/23658
NOTE: Doco for CharFXTransform.xml has changed significantly in 4.0, where terminology has changed to "glyph". Therefore, proposing this change for 3.x branch only.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
See https://github.com/godotengine/godot/pull/23658#issuecomment-562706669
The method was implemented back when Dictionary.get(key, default) did not
exist, but now that it does we do not need a custom method in CharFXTransform.
It's a new feature in 3.2, so does not break compat with 3.1.x.
