editor: Fix panics in CursorPosition::update_position (#41237)

Fixes a regression introduced in
https://github.com/zed-industries/zed/pull/39857. As for the exact
reason this causes this issue I am not yet sure will investigate (as per
the todos in code)

Fixes ZED-23R

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/editor/src/display_map/block_map.rs

@@ -69,6 +69,8 @@ impl From<CustomBlockId> for ElementId {
     }
 }
 
+/// A zero-indexed point in a text buffer consisting of a row and column
+/// adjusted for inserted blocks, wrapped rows, tabs, folds and inlays.
 #[derive(Copy, Clone, Debug, Default, Eq, Ord, PartialOrd, PartialEq)]
 pub struct BlockPoint(pub Point);
 
@@ -80,11 +82,16 @@ struct WrapRow(u32);
 
 pub type RenderBlock = Arc<dyn Send + Sync + Fn(&mut BlockContext) -> AnyElement>;
 
+/// Where to place a block.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum BlockPlacement<T> {
+    /// Place the block above the given position.
     Above(T),
+    /// Place the block below the given position.
     Below(T),
+    /// Place the block next the given position.
     Near(T),
+    /// Replace the given range of positions with the block.
     Replace(RangeInclusive<T>),
 }