Update documentation for "noselect"

In particular, make the distinction and interaction between "noinsert"
and "noselect" clearer as it was very confusing before.

Author: dundargoc

runtime/doc/options.txt

@@ -2155,9 +2155,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 		    a match from the menu. Only works in combination with
 		    "menu" or "menuone". No effect if "longest" is present.
 
-	   noselect Do not select a match in the menu, force the user to
-		    select one from the menu. Only works in combination with
-		    "menu" or "menuone".
+	   noselect Same as "noinsert", except that no menu item is
+		    pre-selected. If both "noinsert" and "noselect" are present,
+		    "noselect" has precedence.
 
 	   fuzzy    Enable |fuzzy-matching| for completion candidates. This
 		    allows for more flexible and intuitive matching, where