Improve guide for Hash#transform_keys and related methods.

[skip ci]

guides/source/active_support_core_extensions.md

@@ -2719,11 +2719,14 @@ The method `transform_keys` accepts a block and returns a hash that has applied
 # => {"" => nil, "A" => :a, "1" => 1}
 ```
 
-The result in case of collision is undefined:
+The result in case of key collision is not guaranteed:
 
 ```ruby
 {"a" => 1, a: 2}.transform_keys { |key| key.to_s.upcase }
-# => {"A" => 2}, in my test, can't rely on this result though
+# The result could either be
+# => {"A"=>2}
+# or
+# => {"A"=>1}
 ```
 
 This method may be useful for example to build specialized conversions. For instance `stringify_keys` and `symbolize_keys` use `transform_keys` to perform their key conversions:
@@ -2758,11 +2761,14 @@ The method `stringify_keys` returns a hash that has a stringified version of the
 # => {"" => nil, "a" => :a, "1" => 1}
 ```
 
-The result in case of collision is undefined:
+The result in case of key collision is not guaranteed:
 
 ```ruby
 {"a" => 1, a: 2}.stringify_keys
-# => {"a" => 2}, in my test, can't rely on this result though
+# The result could either be
+# => {"a"=>2}
+# or
+# => {"a"=>1}
 ```
 
 This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionView::Helpers::FormHelper` defines:
@@ -2799,11 +2805,14 @@ The method `symbolize_keys` returns a hash that has a symbolized version of the
 
 WARNING. Note in the previous example only one key was symbolized.
 
-The result in case of collision is undefined:
+The result in case of key collision is not guaranteed:
 
 ```ruby
 {"a" => 1, a: 2}.symbolize_keys
-# => {:a=>2}, in my test, can't rely on this result though
+# The result could either be
+# => {:a=>2}
+# or
+# => {:a=>1}
 ```
 
 This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionController::UrlRewriter` defines