docs: add deprecation note for 'back' redirect in res-redirect.md and res-location.md v4 (#1835)

Author: bjohansebas

_includes/api/en/4x/res-location.md

@@ -8,6 +8,8 @@ res.location('http://example.com')
 res.location('back')
 ```
 
+{% include admonitions/note.html content="`'back'` was deprecated in 4.21.0, use `req.get('Referrer') || '/'` as an argument instead." %}
+
 A `path` value of "back" has a special meaning, it refers to the URL specified in the `Referer` header of the request. If the `Referer` header was not specified, it refers to "/".
 
 See also [Security best practices: Prevent open redirect 

_includes/api/en/4x/res-redirect.md

@@ -52,5 +52,7 @@ defaulting to `/` when the referer is missing.
 res.redirect('back')
 ```
 
+{% include admonitions/note.html content="`back` redirect was deprecated in 4.21.0, use `req.get('Referrer') || '/'` as an argument instead." %}
+
 See also [Security best practices: Prevent open redirect 
 vulnerabilities](http://expressjs.com/en/advanced/best-practice-security.html#prevent-open-redirects). 

_includes/api/en/5x/res-location.md

@@ -5,14 +5,8 @@ Sets the response `Location` HTTP header to the specified `path` parameter.
 ```js
 res.location('/foo/bar')
 res.location('http://example.com')
-res.location('back')
 ```
 
-A `path` value of "back" has a special meaning, it refers to the URL specified in the `Referer` header of the request. If the `Referer` header was not specified, it refers to "/".
-
-See also [Security best practices: Prevent open redirect 
-vulnerabilities](http://expressjs.com/en/advanced/best-practice-security.html#prevent-open-redirects). 
-
 <div class='doc-box doc-warn' markdown="1">
 After encoding the URL, if not encoded already, Express passes the specified URL to the browser in the `Location` header,
 without any validation.

_includes/api/en/5x/res-redirect.md

@@ -45,12 +45,5 @@ Path-relative redirects are also possible. If you were on
 res.redirect('..')
 ```
 
-A `back` redirection redirects the request back to the [referer](http://en.wikipedia.org/wiki/HTTP_referer),
-defaulting to `/` when the referer is missing.
-
-```js
-res.redirect('back')
-```
-
 See also [Security best practices: Prevent open redirect 
 vulnerabilities](http://expressjs.com/en/advanced/best-practice-security.html#prevent-open-redirects).