Rust: Update doc comments for StmtList.

Author: geoffw0

rust/schema/annotations.py

@@ -1828,13 +1828,15 @@ class _:
 @annotate(StmtList)
 class _:
     """
-    A list of statements in a block.
+    A list of statements in a block, with an optional tail expression at the
+    end that determines the block's value.
 
     For example:
     ```rust
     {
         let x = 1;
         let y = 2;
+        x + y
     }
     //  ^^^^^^^^^
     ```