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/fold_map.rs

@@ -178,6 +178,9 @@ impl<'a> FoldMapWriter<'a> {
     }
 }
 
+/// Decides where the fold indicators should be; also tracks parts of a source file that are currently folded.
+///
+/// See the [`display_map` module documentation](crate::display_map) for more information.
 pub(crate) struct FoldMap {
     snapshot: FoldSnapshot,
     ellipses_color: Option<Hsla>,