Update src/tag_gc.rs

Co-authored-by: Ralf Jung <[email protected]>

src/tag_gc.rs

@@ -7,6 +7,8 @@ use crate::*;
 pub type TagVisitor<'a> = dyn FnMut(Option<AllocId>, Option<BorTag>) + 'a;
 
 pub trait VisitTags {
+    /// Will be called on all pointers in the state, as well as any
+    /// other place where we store `AllocId` and/or `BorTag`.
     fn visit_tags(&self, visit: &mut TagVisitor<'_>);
 }