Update Rack::Head documentation to match code behavior

In commit 449039a,
ActionDispatch::Head was replaced with Rack::Head in the
middleware. However, the documentation was not changed.

Rack::Head code behaviour is not to change HEAD into GET it is to
close the body of all HEAD requests. This commit fixes the docs
to show this.

The left-over ActionDisplayt::Head documentation is,
unsurprisingly, wrong. Rack::Head code behaviour is not to change
HEAD into GET it is to close the body of all HEAD requests.

This commit fixes the docs to show this.

Author: notapatch

guides/source/configuring.md

@@ -840,7 +840,7 @@ Allows the method to be overridden if `params[:_method]` is set. This is the mid
 
 #### `Rack::Head`
 
-Converts HEAD requests to GET requests and serves them as so.
+Returns an empty body for all HEAD requests. It leaves all other requests unchanged.
 
 #### Adding Custom Middleware
 

guides/source/rails_on_rack.md

@@ -339,7 +339,7 @@ Much of Action Controller's functionality is implemented as Middlewares. The fol
 
 **`Rack::Head`**
 
-* Converts HEAD requests to `GET` requests and serves them as so.
+* Returns an empty body for all HEAD requests. It leaves all other requests unchanged.
 
 **`Rack::ConditionalGet`**