add before and after

Author: mpeddada1

doc/v3-migration-guide.md

@@ -53,6 +53,14 @@ module which can be added to your `MODULE.bazel` file as a dependency.
 AsyncRowReader::NO_ROWS_LIMIT has been removed. Please use
 `google::cloud::bigtable::RowReader::NO_ROWS_LIMIT` instead.
 
+```cpp
+// Before
+auto limit = google::cloud::bigtable::AsyncRowReader<>::NO_ROWS_LIMIT;
+
+// After
+auto limit = google::cloud::bigtable::RowReader::NO_ROWS_LIMIT;
+```
+
 </details>
 
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