Better copy and deprecation message

Address comments by @kaspth

Author: schneems

README.md

@@ -52,7 +52,7 @@ Also see [Sprockets::Rails::Task](https://github.com/rails/sprockets-rails/blob/
 
 **`config.assets.unknown_asset_fallback`**
 
-When set to a truthy value, the a result will be returned even if the requested asset is not found in the asset pipeline. When set to `false` it will raise an error.
+When set to a truthy value, a result will be returned even if the requested asset is not found in the asset pipeline. When set to a falsey value it will raise an error when no asset is found in the pipeline. Defaults to `true`.
 
 **`config.assets.precompile`**
 

lib/sprockets/rails/helper.rb

@@ -7,6 +7,7 @@ module Sprockets
   module Rails
     module Helper
       class AssetNotFound < StandardError; end
+
       class AssetNotPrecompiled < StandardError
         include Sprockets::Rails::Utils
         def initialize(source)
@@ -82,8 +83,10 @@ def compute_asset_path(path, options = {})
           raise AssetNotFound, message unless unknown_asset_fallback
 
           if respond_to?(:public_compute_asset_path)
-            message << "The public fallback behavior is being deprecaed and will be removed.\n"
-            message << "pass in `skip_pipeline: true` instead.\n"
+            message << "Falling back to an asset that may be in the public folder.\n"
+            message << "This behavior is deprecated and will be removed.\n"
+            message << "To bypass the asset pipeline and preserve this behavior,\n"
+            message << "use the `skip_pipeline: true` option.\n"
 
             call_stack = respond_to?(:caller_locations) ? caller_locations : caller
             ActiveSupport::Deprecation.warn(message, call_stack)