Clarify the documentation of the YAML module [Misc #14567]

Based on a patch from Victor Shepelev

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62635 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/yaml.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: false
-##
-# The YAML module is an alias of Psych, the YAML engine for Ruby.
 
 begin
   require 'psych'
@@ -17,7 +15,7 @@
 #
 # This module provides a Ruby interface for data serialization in YAML format.
 #
-# The underlying implementation is the libyaml wrapper Psych.
+# The YAML module is an alias of Psych, the YAML engine for Ruby.
 #
 # == Usage
 #
@@ -31,6 +29,9 @@
 #     YAML.dump("foo")     # => "--- foo\n...\n"
 #     { :a => 'b'}.to_yaml  # => "---\n:a: b\n"
 #
+# As the implementation is provided by the Psych library, detailed documentation
+# can be found in that library's docs (also part of standard library).
+#
 # == Security
 #
 # Do not use YAML to load untrusted data. Doing so is unsafe and could allow