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/text/src/anchor.rs

@@ -6,10 +6,13 @@ use anyhow::Result;
 use std::{cmp::Ordering, fmt::Debug, ops::Range};
 use sum_tree::Bias;
 
+/// A timestamped position in a buffer
 #[derive(Copy, Clone, Eq, PartialEq, Debug, Hash, Default)]
 pub struct Anchor {
     pub timestamp: clock::Lamport,
+    /// The byte offset in the buffer
     pub offset: usize,
+    /// Describes which character the anchor is biased towards
     pub bias: Bias,
     pub buffer_id: Option<BufferId>,
 }