Editorial adition to the note about the spatial navigation starting point

Closes w3c#23

Author: frivoal

index.bs

@@ -996,7 +996,8 @@ Navigation</h3>
 There can be a <dfn>spatial navigation starting point</dfn>. It is initially unset.
 The user agent may set it when the user indicates that it should be moved.
 
-Note: For example, the user agent could set it to the position of the user's click if the user clicks on the document contents.
+Note: For example, the user agent could set it to the position of the user's click if the user clicks on the document contents,
+and unset when the focus is moved (by spatial navigation or any other means).
 
 If the UA sets both a <a>spatial navigation starting point</a> and a <a>sequential focus navigation starting point</a>,
 they must not be set differently.