Add note about human_attribute_name symbol/string

This was always the intended API as per discussion in https://github.com/rails/rails/issues/36916, however versions before Rails 6 always passed a symbol. This means that some apps relied on symbols to be passed, this change is intended to note the change in behaviour so that apps can respond in kind.

activemodel/CHANGELOG.md

@@ -89,6 +89,16 @@
 
 ## Rails 6.0.0.beta1 (January 18, 2019) ##
 
+*   Internal calls to `human_attribute_name` on an `Active Model` now pass attributes as strings instead of symbols
+    in some cases.
+
+    This is in line with examples in Rails docs and puts the code in line with the intention -
+    the potential use of strings or symbols.
+
+    It is recommended to cast the attribute input to your desired type as if you you are overriding that methid.
+
+    *Martin Larochelle*
+
 *   Add `ActiveModel::Errors#of_kind?`.
 
     *bogdanvlviv*, *Rafael Mendonça França*