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/tab_map.rs

@@ -9,6 +9,9 @@ use sum_tree::Bias;
 
 const MAX_EXPANSION_COLUMN: u32 = 256;
 
+/// Keeps track of hard tabs in a text buffer.
+///
+/// See the [`display_map` module documentation](crate::display_map) for more information.
 pub struct TabMap(TabSnapshot);
 
 impl TabMap {