[io grant] Document IO::Handle.gist

Rakudo impl: rakudo/rakudo@276d4a7e7e
Tests: https://github.com/perl6/roast/blob/a3410d1143/integration/advent2010-day03.t#L29-L36

Author: zoffixznet

doc/Type/IO/Handle.pod6

@@ -85,6 +85,21 @@ should close the handles explicitly as well, regardless of whether they were
 open for writing, since too many files might get opened before garbage
 collection happens and the no longer used handles get closed.
 
+=head method gist
+
+Defined as:
+
+    method gist(IO::Handle:D: --> Str:D)
+
+Returns a string containing information which
+L«C<.path>|/type/IO::Handle#method_path», if any, the handle is created for
+and whether it is L«C<.opened>|/type/IO::Handle#method_opened».
+
+=begin code :skip-test
+    say IO::Handle.new; # IO::Handle<(Any)>(closed)
+    say "foo".IO.open;  # IO::Handle<"foo".IO>(opened)
+=end code
+
 =head2 method eof
 
 Defined as: