Rust: Fix grammar and add documentation comments

Author: paldepind

rust/ql/lib/codeql/rust/controlflow/internal/Scope.qll

@@ -15,10 +15,12 @@ abstract private class CfgScopeImpl extends AstNode {
 final class CfgScope = CfgScopeImpl;
 
 final class AsyncBlockScope extends CfgScopeImpl, AsyncBlockExpr {
+  /* Holds if `first` is executed first when entering `scope`. */
   override predicate scopeFirst(AstNode first) {
     first(this.(ExprTrees::AsyncBlockExprTree).getFirstChildNode(), first)
   }
 
+  /* Holds if `scope` is exited when `last` finishes with completion `c`. */
   override predicate scopeLast(AstNode last, Completion c) {
     last(this.(ExprTrees::AsyncBlockExprTree).getLastChildElement(), last, c)
   }

rust/ql/lib/codeql/rust/elements/AsyncBlockExpr.qll

@@ -1,7 +1,11 @@
+/**
+ * This module provides the public class `AsyncBlockExpr`.
+ */
+
 private import codeql.rust.elements.BlockExpr
 
 /**
- * A async block expression. For example:
+ * An async block expression. For example:
  * ```rust
  * async {
  *     let x = 42;