docs: clarify req.params type and usage with regular expressions

Fixes #2100

Author: krzysdz

_includes/api/en/4x/req-params.md

@@ -8,14 +8,16 @@ console.dir(req.params.name)
 // => 'tj'
 ```
 
-When you use a regular expression for the route definition, capture groups are provided in the array using `req.params[n]`, where `n` is the n<sup>th</sup> capture group. This rule is applied to unnamed wild card matches with string routes such as `/file/*`:
+When you use a regular expression for the route definition, capture groups are provided as integer string keys using `req.params['n']`, where `n` is the n<sup>th</sup> capture group. This rule is applied to unnamed wild card matches with string routes such as `/file/*`:
 
 ```js
 // GET /file/javascripts/jquery.js
-console.dir(req.params[0])
+console.dir(req.params['0'])
 // => 'javascripts/jquery.js'
 ```
 
+Named capturing groups in regular expressions behave like named route parameters. For example the group from `/^\/file\/(?<path>.*)$/` expression is available as `req.params.path`.
+
 If you need to make changes to a key in `req.params`, use the [app.param](/{{ page.lang }}/4x/api.html#app.param) handler. Changes are applicable only to [parameters](/{{ page.lang }}/guide/routing.html#route-parameters) already defined in the route path.
 
 Any changes made to the `req.params` object in a middleware or route handler will be reset.

_includes/api/en/5x/req-params.md

@@ -8,16 +8,18 @@ console.dir(req.params.name)
 // => "tj"
 ```
 
-When you use a regular expression for the route definition, capture groups are provided in the array using `req.params[n]`, where `n` is the n<sup>th</sup> capture group.
+When you use a regular expression for the route definition, capture groups are provided as integer string keys using `req.params['n']`, where `n` is the n<sup>th</sup> capture group.
 
 ```js
 app.use(/^\/file\/(.*)$/, (req, res) => {
   // GET /file/javascripts/jquery.js
-  console.dir(req.params[0])
+  console.dir(req.params['0'])
   // => "javascripts/jquery.js"
 })
 ```
 
+Named capturing groups in regular expressions behave like named route parameters. For example the group from `/^\/file\/(?<path>.*)$/` expression is available as `req.params.path`.
+
 If you need to make changes to a key in `req.params`, use the [app.param](/{{ page.lang }}/5x/api.html#app.param) handler. Changes are applicable only to [parameters](/{{ page.lang }}/guide/routing.html#route-parameters) already defined in the route path.
 
 Any changes made to the `req.params` object in a middleware or route handler will be reset.