Fix editor::SplitSelectionIntoLines adding an extra line at the end (#25053)

Closes #4795

Release Notes:

- Fixed `editor::SplitSelectionIntoLines` adding an extra line at end
of selection

crates/rope/src/rope.rs

@@ -975,7 +975,10 @@ pub struct TextSummary {
     pub chars: usize,
     /// Length in UTF-16 code units
     pub len_utf16: OffsetUtf16,
-    /// A point representing the number of lines and the length of the last line
+    /// A point representing the number of lines and the length of the last line.
+    ///
+    /// In other words, it marks the point after the last byte in the text, (if
+    /// EOF was a character, this would be its position).
     pub lines: Point,
     /// How many `char`s are in the first line
     pub first_line_chars: u32,