doc: add require.main to require properties

`require.main` was documented in a non-standard way.

With this PR, the previous section is left as is
to not break all the possible link references
inside and outside Node.js docs.

A standard section is added to the `require` properties
with a reference to the old description.

PR-URL: nodejs#19573
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/modules.md

@@ -600,6 +600,43 @@ filename scales linearly with the number of registered extensions.
 In other words, adding extensions slows down the module loader and
 should be discouraged.
 
+#### require.main
+<!-- YAML
+added: v0.1.17
+-->
+
+* {Object}
+
+The `Module` object representing the entry script loaded when the Node.js
+process launched.
+See ["Accessing the main module"](#modules_accessing_the_main_module).
+
+In `entry.js` script:
+
+```js
+console.log(require.main);
+```
+
+```sh
+node entry.js
+```
+
+<!-- eslint-skip -->
+```js
+Module {
+  id: '.',
+  exports: {},
+  parent: null,
+  filename: '/absolute/path/to/entry.js',
+  loaded: false,
+  children: [],
+  paths:
+   [ '/absolute/path/to/node_modules',
+     '/absolute/path/node_modules',
+     '/absolute/node_modules',
+     '/node_modules' ] }
+```
+
 #### require.resolve(request[, options])
 <!-- YAML
 added: v0.3.0