[io grant] Document IO::Spec::*.rootdir

Author: zoffixznet

doc/Type/IO/Spec/Unix.pod6

@@ -235,6 +235,14 @@ are the same.
     # OUTPUT: «/home/camelia/bar/foo/../..␤»
 =end code
 
+=head2 method rootdir
+
+Defined as:
+
+    method rootdir(--> Str:D)
+
+Returns string C<'/'>, representing root directory.
+
 =head2 method split
 
 Defined as:

doc/Type/IO/Spec/Win32.pod6

@@ -207,6 +207,14 @@ are the same.
     # OUTPUT: «C:\Users\camelia\bar\foo\..\..␤»
 =end code
 
+=head2 method rootdir
+
+Defined as:
+
+    method rootdir(--> Str:D)
+
+Returns string C<｢\｣>, representing root directory.
+
 =head2 method split
 
 Defined as: