[io grant] Document IO::Path.sibling

Rakudo impl: rakudo/rakudo@8bacad8f84
Tests: Raku/roast@3b36d4db0d

Author: zoffixznet

doc/Type/IO/Path.pod6

@@ -473,6 +473,23 @@ is dependant on the operating system. The method will L«C<fail>|/routine/fail»
 with C<X::IO::DoesNotExist> if the path points to a non-existent filesystem
 entity.
 
+=head2 method sibling
+
+    method sibling(IO::Path:D: Str() $sibling --> IO::Path:D)
+
+Allows to reference a sibling file or directory. Returns a new
+L«C<IO::Path>|/type/IO::Path» based on the invocant, with the
+# L«C<.basename>|/type/IO::Path#method_basename» changed to C<$sibling>. The
+C<$sibling> is allowed to be a multi-part path fragment, although
+# L«C<.add>|/type/IO::Path#method_add» is a better choice for such use.
+
+    say '.bashrc'.IO.sibling: '.bash_aliases'; # OUTPUT: «.bash_aliases".IO␤»
+    say '/home/camelia/.bashrc'.IO.sibling: '.bash_aliases';
+    # OUTPUT: «/home/camelia/.bash_aliases".IO␤»
+
+    say '/foo/' .IO.sibling: 'bar';  # OUTPUT: «/bar".IO␤»
+    say '/foo/.'.IO.sibling: 'bar';  # OUTPUT: «/foo/bar".IO␤»
+
 =head2 routine slurp
 
     multi method slurp(IO::Path:D: :$bin, :$enc)