Add documentation to many core editor types (#7919)

Hopefully this makes it a bit easier for new contributors to dive into
the codebase :)

Release Notes:

- Improved documentation for many core editor types

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/editor/src/display_map/block_map.rs

@@ -22,6 +22,9 @@ use text::Edit;
 
 const NEWLINES: &[u8] = &[b'\n'; u8::MAX as usize];
 
+/// Tracks custom blocks such as diagnostics that should be displayed within buffer.
+///
+/// See the [`display_map` module documentation](crate::display_map) for more information.
 pub struct BlockMap {
     next_block_id: AtomicUsize,
     wrap_snapshot: RefCell<WrapSnapshot>,