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: https://github.com/nodejs/node/pull/19573


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