RTL: Clarify that line, paragraph, and character numbers are zero-indexed

[akien-mga]

Closes #82885 once cherry-picked to 4.1 with this additional change:

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 895d4da342..52395a3037 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -598,6 +598,7 @@ See the [release announcement](https://godotengine.org/article/maintenance-relea
 #### GUI
 
 - Fix RichTextLabel character line and paragraph index getters ([GH-76759](https://github.com/godotengine/godot/pull/76759)).
+  * This corrects an off-by-one error for the indices, making lines/paragraphs zero-indexed instead of one-indexed.
 - Fix text overlapping icon in `Tree` ([GH-78756](https://github.com/godotengine/godot/pull/78756)).
 - Fix delay on tab resizing when (un)hovering tabs ([GH-78777](https://github.com/godotengine/godot/pull/78777)).
 - Fix `Tree` performance regression by using cache ([GH-79325](https://github.com/godotengine/godot/pull/79325)).

[akien-mga]

Cherry-picked for 4.2.2.
Cherry-picked for 4.1.4.