Add docs clarifying that FieldDescriptions may return nil

https://github.com/jackc/pgx/issues/1634

rows.go

@@ -28,12 +28,16 @@ type Rows interface {
 	// to call Close after rows is already closed.
 	Close()
 
-	// Err returns any error that occurred while reading.
+	// Err returns any error that occurred while reading. Err must only be called after the Rows is closed (either by
+	// calling Close or by Next returning false). If it is called early it may return nil even if there was an error
+	// executing the query.
 	Err() error
 
 	// CommandTag returns the command tag from this query. It is only available after Rows is closed.
 	CommandTag() pgconn.CommandTag
 
+	// FieldDescriptions returns the field descriptions of the columns. It may return nil. In particular this can occur
+	// when there was an error executing the query.
 	FieldDescriptions() []pgconn.FieldDescription
 
 	// Next prepares the next row for reading. It returns true if there is another