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,

crates/rope/src/rope.rs

@@ -953,15 +953,24 @@ impl sum_tree::Summary for ChunkSummary {
     }
 }
 
+/// Summary of a string of text.
 #[derive(Clone, Debug, Default, Eq, PartialEq)]
 pub struct TextSummary {
+    /// Length in UTF-8
     pub len: usize,
+    /// Length in UTF-16 code units
     pub len_utf16: OffsetUtf16,
+    /// A point representing the number of lines and the length of the last line
     pub lines: Point,
+    /// How many `char`s are in the first line
     pub first_line_chars: u32,
+    /// How many `char`s are in the last line
     pub last_line_chars: u32,
+    /// How many UTF-16 code units are in the last line
     pub last_line_len_utf16: u32,
+    /// The row idx of the longest row
     pub longest_row: u32,
+    /// How many `char`s are in the longest row
     pub longest_row_chars: u32,
 }