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/rope/src/point.rs

@@ -3,6 +3,7 @@ use std::{
     ops::{Add, AddAssign, Sub},
 };
 
+/// A zero-indexed point in a text buffer consisting of a row and column.
 #[derive(Clone, Copy, Default, Eq, PartialEq, Debug, Hash)]
 pub struct Point {
     pub row: u32,