documentation on source_root is misleading

### Standalone code, or other way to reproduce the problem

Given the code:

```php
hhvm.server.source_root
> working directory of HHVM process
> For server mode, this will hold the path to the root of the directory of the code being served up. This setting is _useless_ in repo-authoritative mode.

But this setting affects `__DIR__` even in repo-authoritative mode.