Document how to write named arguments that do not create any symbols

doc/Type/Signature.pod6

@@ -885,6 +885,16 @@ say alias-named(color => "red", style => "A");
 say alias-named(colour => "green", variety => "B");
 say alias-named(color => "white", class => "C");
 
+Using argument aliases, you can create named arguments that do not
+create any symbols. This is useful when using named arguments as
+sentinels:
+
+=for code
+multi sub trait_mod:<is>(Routine:D $r is raw, :timestamped($)!) {
+    $r does role timestamped { has Instant:D @.timestamps };
+    $r.wrap: -> | { ENTER $r.timestamps.push: now; callsame };
+}
+
 X<|optional argument>
 =head2 Optional and mandatory arguments
 

xt/words.pws

@@ -1292,6 +1292,9 @@ tiebreaking
 timespec
 timezoneclash
 timezones
+timestamp
+timestamps
+timestamped
 timtoady
 timtowtdi
 titlecase