Update the guide with range requests support

Also update the list of headers cowboy_rest might set
and tweak a small number of other items.

doc/src/guide/resource_design.asciidoc

@@ -144,6 +144,16 @@ never be called.
 Implement the `languages_provided` or `charsets_provided`
 callbacks if applicable.
 
+Does the resource accept ranged requests? If it does,
+implement the `ranges_provided` callback. Resources that
+only accept `bytes` units can use the callback name
+`auto` and let Cowboy automatically do ranged responses.
+Other callbacks should have a name prefix of `ranged_`
+for clarity. For example, `ranged_bytes` or `ranged_pages`.
+If the resource needs to perform additional checks before
+accepting to do a ranged responses, implement the
+`range_satisfiable` callback.
+
 Is there any other header that may make the representation
 of the resource vary? Implement the `variances` callback.
 
@@ -191,10 +201,15 @@ the `options` method.
 === GET and HEAD methods
 
 If you implement the methods GET and/or HEAD, you must
-implement one `ProvideResource` callback for each
+implement one `ProvideCallback` callback for each
 content-type returned by the `content_types_provided`
 callback.
 
+When range requests are accepted, you must implement one
+`RangeCallback` for each range unit returned by
+`ranges_provided` (unless `auto` was used). This is
+in addition to the `ProvideCallback` callback.
+
 === PUT, POST and PATCH methods
 
 If you implement the methods PUT, POST and/or PATCH,

doc/src/guide/rest_flowcharts.asciidoc

@@ -95,6 +95,11 @@ callback will only be called at the end of the
 "GET and HEAD methods" diagram, when all conditions
 have been met.
 
+Optionally, the `ranges_provided` also returns the
+name of a callback for every range unit it accepts. This
+will be called at the end of the "GET and HEAD methods"
+diagram in the case of ranged requests.
+
 The selected content-type, language and charset are
 saved as meta values in the Req object. You *should*
 use the appropriate representation if you set a
@@ -121,11 +126,18 @@ succeed, the resource can be retrieved.
 
 Cowboy prepares the response by first retrieving
 metadata about the representation, then by calling
-the `ProvideResource` callback. This is the callback
+the `ProvideCallback` callback. This is the callback
 you defined for each content-types you returned from
 `content_types_provided`. This callback returns the body
-that will be sent back to the client, or a fun if the
-body must be streamed.
+that will be sent back to the client.
+
+For ranged requests, but only when the `ranges_provided`
+callback was defined earlier, Cowboy will add the selected
+`range` information to the Req object and call the
+`range_satisfiable` callback. After confirming that the
+range can be provided, Cowboy calls the `RangeResource`
+callback and produces a ranged response using the
+ranged data from the callback.
 
 When the resource does not exist, Cowboy will figure out
 whether the resource existed previously, and if so whether