See discussion in #1547 for context.

The name modifications is intended to map to the RPC method name ReadModifyWriteRow but the word modification is a synonym of mutation, so this method is poorly named. The method actually sends changes that

• increment an int (stored in BT as 8 bytes)
• append bytes to a cell value

so we should either

• Fold it into Row.commit() as commit(append_only=True)
• Rename it to something like Row.commit_cell_updates (or a better name than that)