doc: fix modules require.resolve documentation

The behavior of the `node_modules` lookup algorithm was
changed in #1177, but the documentation was not updated completely
to describe the new behavior.

The pseudocode of the lookup algorithm did not metion that
`index.json` is tried to be loaded if you require a folder.

Reviewed-By: Fedor Indutny <fedor@indutny.com>
doc: document `process.env` better

Fixes #6424.

Reviewed-By: Fedor Indutny <fedor@indutny.com>