git: Use doc comments for ResetMode variants (#27882)

This PR updates the `ResetMode` enum to use doc comments for its
variants instead of line comments.

Release Notes:

- N/A

crates/git/src/repository.rs

@@ -161,13 +161,11 @@ pub struct Remote {
 }
 
 pub enum ResetMode {
-    // reset the branch pointer, leave index and worktree unchanged
-    // (this will make it look like things that were committed are now
-    // staged)
+    /// Reset the branch pointer, leave index and worktree unchanged (this will make it look like things that were
+    /// committed are now staged).
     Soft,
-    // reset the branch pointer and index, leave worktree unchanged
-    // (this makes it look as though things that were committed are now
-    // unstaged)
+    /// Reset the branch pointer and index, leave worktree unchanged (this makes it look as though things that were
+    /// committed are now unstaged).
     Mixed,
 }