Update documentation

Author: remear

docs/general/key_transform.md renamed to docs/general/transform.md

@@ -55,6 +55,9 @@ def include_meta(json)
       end
 
       class << self
+        # Sets the default transform for the adapter.
+        #
+        # @return [Symbol] the default transform for the adapter
         def default_transform
           :unaltered
         end
@@ -69,6 +72,11 @@ def transform(options)
           ActiveModelSerializers.config.transform || default_transform
         end
 
+        # Transforms the casing of the supplied value.
+        #
+        # @param value [Object] the value to be transformed
+        # @param options [Object] serializable resource options
+        # @return [Symbol] the default transform for the adapter
         def transform_casing!(value, options)
           Transform.send(transform(options), value)
         end