Update ComboBox.xml (#4529)

Add a remark that reading `ComboBox.Text` is dependent on whether there is a selected item or not.

Author: RussKie

xml/System.Windows.Forms/ComboBox.xml

@@ -3919,7 +3919,9 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- Setting the <xref:System.Windows.Forms.ComboBox.Text%2A> property to `null` or an empty string ("") sets the <xref:System.Windows.Forms.ComboBox.SelectedIndex%2A> to -1. Setting the <xref:System.Windows.Forms.ComboBox.Text%2A> property to a value that is in the <xref:System.Windows.Forms.ComboBox.Items%2A> collection sets the <xref:System.Windows.Forms.ComboBox.SelectedIndex%2A> to the index of that item. Setting the <xref:System.Windows.Forms.ComboBox.Text%2A> property to a value that is not in the collection leaves the <xref:System.Windows.Forms.ComboBox.SelectedIndex%2A> unchanged.  
+ Setting the <xref:System.Windows.Forms.ComboBox.Text%2A> property to `null` or an empty string ("") sets the <xref:System.Windows.Forms.ComboBox.SelectedIndex%2A> to -1. Setting the <xref:System.Windows.Forms.ComboBox.Text%2A> property to a value that is in the <xref:System.Windows.Forms.ComboBox.Items%2A> collection sets the <xref:System.Windows.Forms.ComboBox.SelectedIndex%2A> to the index of that item. Setting the <xref:System.Windows.Forms.ComboBox.Text%2A> property to a value that is not in the collection leaves the <xref:System.Windows.Forms.ComboBox.SelectedIndex%2A> unchanged.
+
+Reading the <xref:System.Windows.Forms.ComboBox.Text%2A> property returns the text of <xref:System.Windows.Forms.ComboBox.SelectedItem%2A>, if it is not `null`.