Document that ast::Variable::type can be null

Change-Id: I7a29c1de49cbd082a5c3e0fc4bdcdead9dd2c956
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/70340
Auto-Submit: David Neto <dneto@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: David Neto <dneto@google.com>

src/ast/variable.h

@@ -135,7 +135,9 @@ class Variable : public Castable<Variable, Node> {
   /// The variable symbol
   const Symbol symbol;
 
-  /// The variable type
+  /// The declared variable type. This is null if the type is inferred, e.g.:
+  ///   let f = 1.0;
+  ///   var i = 1;
   const ast::Type* const type;
 
   /// True if this is a constant, false otherwise