[io grant] Document IO::Spec::Unix.updir

doc/Type/IO/Spec/Unix.pod6

@@ -121,6 +121,16 @@ Defined as:
 
 Attempts to locate a system's temporary directory by checking several typical directories and environmental variables. Uses current directory if no suitable directories are found.
 
+=head2 method updir
+
+Defined as:
+
+    method updir()
+
+Returns a string representing the directory one up from current:
+
+    say '..' eq $*SPEC.updir; # OUTPUT: «True␤»
+
 =end pod
 
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