Prefer official Ruby documentation

Author: byroot

guides/source/2_3_release_notes.md

@@ -503,7 +503,7 @@ A lot of folks have adopted the notion of using try() to attempt operations on o
 
 ### Object#tap Backport
 
-`Object#tap` is an addition to [Ruby 1.9](http://www.ruby-doc.org/core-1.9/classes/Object.html#M000309) and 1.8.7 that is similar to the `returning` method that Rails has had for a while: it yields to a block, and then returns the object that was yielded. Rails now includes code to make this available under older versions of Ruby as well.
+`Object#tap` is an addition to [Ruby 1.9](https://docs.ruby-lang.org/en/master/Kernel.html#method-i-tap) and 1.8.7 that is similar to the `returning` method that Rails has had for a while: it yields to a block, and then returns the object that was yielded. Rails now includes code to make this available under older versions of Ruby as well.
 
 ### Swappable Parsers for XMLmini
 

guides/source/4_1_release_notes.md

@@ -338,7 +338,7 @@ for detailed changes.
   array. ([Pull Request](https://github.com/rails/rails/pull/9599))
 
 * Added `session#fetch` method fetch behaves similarly to
-  [Hash#fetch](https://www.ruby-doc.org/core-1.9.3/Hash.html#method-i-fetch),
+  [Hash#fetch](https://docs.ruby-lang.org/en/master/Hash.html#method-i-fetch),
   with the exception that the returned value is always saved into the
   session. ([Pull Request](https://github.com/rails/rails/pull/12692))
 

guides/source/upgrading_ruby_on_rails.md

@@ -2151,9 +2151,10 @@ If your application currently depends on MultiJSON directly, you have a few opti
 
 2. Migrate away from MultiJSON by using `obj.to_json`, and `JSON.parse(str)` instead.
 
-WARNING: Do not simply replace `MultiJson.dump` and `MultiJson.load` with
-`JSON.dump` and `JSON.load`. These JSON gem APIs are meant for serializing and
-deserializing arbitrary Ruby objects and are generally [unsafe](https://ruby-doc.org/stdlib-2.2.2/libdoc/json/rdoc/JSON.html#method-i-load).
+WARNING: Do not simply replace `MultiJson.load` with
+`JSON.load` as that API is meant for deserializing arbitrary Ruby objects and
+is generally [unsafe](https://docs.ruby-lang.org/en/master/JSON.html#method-i-load).
+Prefer to use `JSON.parse`.
 
 #### JSON gem compatibility