Document usage of scope before definitions.

Note that before the last commit, the example did not assemble.

doc/ca65.sgml

@@ -1319,6 +1319,11 @@ if you want to access the "other" symbol <tt/bar/, you would have to write:
         .endscope
 </verb></tscreen>
 
+The above example also shows that to search a scope that is not yet defined in
+the code above the usage, you must use the namespace token (<tt/::/) and
+specify the full scope name, to allow the assembler to create the scope at the
+correct place.
+
 
 <sect>Address sizes and memory models<label id="address-sizes"><p>