Document the name of the config file

[webknjaz]

What's the problem this feature will solve?

Docs only say "virtualenv looks for a standard ini configuration file": https://virtualenv.pypa.io/en/latest/cli_interface.html. But there's not a single mention of how that "standard ini file" is named.

Describe the solution you'd like

The file name should be documented.

Alternative Solutions

Read the source code?

Additional context

N/A

[gaborbernat]

virtualenv -help would print it... but we should document on the web page.