[ruby/set] Adding section: What's Here

https://github.com/ruby/set/commit/8f4c62768d

lib/set.rb

@@ -64,7 +64,8 @@
 #
 # ## What's Here
 #
-# First, what's elsewhere. \Set includes the module Enumerable,
+# First, what's elsewhere. \Set includes the module
+# {Enumerable}[https://docs.ruby-lang.org/en/master/Enumerable.html],
 # which provides dozens of additional methods.
 #
 # In particular, class \Set does not have many methods of its own