Document that SearchDelegate.buildResults can be called multiple time… (#26785)

packages/flutter/lib/src/material/search.dart

@@ -108,6 +108,10 @@ abstract class SearchDelegate<T> {
   /// The current value of [query] can be used to determine what the user
   /// searched for.
   ///
+  /// This method might be applied more than once to the same query.
+  /// If your [buildResults] method is computationally expensive, you may want
+  /// to cache the search results for one or more queries.
+  ///
   /// Typically, this method returns a [ListView] with the search results.
   /// When the user taps on a particular search result, [close] should be called
   /// with the selected result as argument. This will close the search page and