Adds "defined" to Junction

Refs Raku#2703 (because it wasn't there) and also Raku#2632 (because it
documents autothreading)

doc/Type/Junction.pod6

@@ -200,7 +200,6 @@ this case on a Junction) and the result will be a C<Bool>.
 
 Defined as:
 
-
     multi method new(Junction: \values, Str :$type!)
     multi method new(Junction: Str:D \type, \values)
 
@@ -210,6 +209,25 @@ a set of values.
     my $j = Junction.new(<Þor Oðinn Loki>, type => "all");
     my $n = Junction.new( "one", 1..6 )
 
+=head2 method defined
+
+Defined as:
+
+    multi method defined(Junction:D:)
+
+Checks for definedness instead of Boolean values.
+
+    say ( 3 | Str).defined ;   # OUTPUT: «True␤»
+    say (one 3, Str).defined;  # OUTPUT: «True␤»
+    say (none 3, Str).defined; # OUTPUT: «False␤»
+
+C<Failure>s are also considered non-defined:
+
+    my $foo=Failure.new;
+    say (one 3, $foo).defined; # OUTPUT: «True␤»
+
+Since 6.d, this method will autothread.
+
 =head2 method Bool
 
 Defined as:
@@ -249,8 +267,8 @@ Defined as:
 
     multi method gist(Junction:D:)
 
-Collapses the L<Junction|/type/Junction> and returns a L<Str|/type/Str> composed of the type of the junction and the
-L<gists|/routine/gist> of its components:
+Collapses the L<Junction|/type/Junction> and returns a L<Str|/type/Str> composed
+of the type of the junction and the L<gists|/routine/gist> of its components:
 
     <a 42 c>.all.say; # OUTPUT: «all(a, 42, c)␤»