include more detailed examples

Author: mpeddada1

doc/v3-migration-guide.md

@@ -56,6 +56,52 @@ have been removed.
 Developers that read rows by directly constructing a `RowReader` object should
 instead construct a `Table` object and call `Table::ReadRows(...)`.
 
+For example, code that used to look like this:
+
+**Before:**
+
+```cpp
+#include "google/cloud/bigtable/data_client.h"
+#include "google/cloud/bigtable/row_reader.h"
+#include "google/cloud/bigtable/table.h"
+
+// ...
+
+auto client = google::cloud::bigtable::MakeDataClient(
+    "my-project", "my-instance", creds);
+auto reader = google::cloud::bigtable::RowReader(
+    client, "my-table-id", google::cloud::bigtable::RowSet("r1", "r2"),
+    google::cloud::bigtable::RowReader::NO_ROWS_LIMIT,
+    google::cloud::bigtable::Filter::PassAllFilter(),
+    /*...retry and backoff policies...*/);
+
+for (auto& row : reader) {
+  if (!row) throw std::move(row).status();
+  // ...
+}
+```
+
+Should be changed to this:
+
+**After:**
+
+```cpp
+#include "google/cloud/bigtable/table.h"
+
+// ...
+
+namespace cbt = google::cloud::bigtable;
+cbt::Table table(cbt::MakeDataConnection(),
+                 cbt::TableResource("my-project", "my-instance", "my-table-id"));
+
+for (auto& row : table.ReadRows(
+         cbt::RowSet("r1", "r2"),
+         cbt::Filter::PassAllFilter())) {
+  if (!row) throw std::move(row).status();
+  // ...
+}
+```
+
 </details>
 
 ### Pubsub