Docker install guide changes

[FozzieHi]

What does this do?

• Change setup arguments - This is through my limited understanding of scripts, so please correct me if I'm wrong! In the docker/setup script, data_reset is defined as data_reset="${1:-false}". This means that data_reset is either the first command-line argument if present, or false otherwise. So in order to run the migration, sample data and xapian index you would have to specify ./docker/setup true. Given the current description in the installation guide is to do this, I think we should include the command line argument.
• Adds a section on how to visit the MailHog instance.
• Minor grammar fixes.

Why was this needed?

To solve issues with databases after Docker installation, and to provide more information in the docs.

[gbp]

Thanks for this. Not sure how I didn't catch this before.

I will change the setup script so passing in true isn't necessary.

[gbp]

Fixed setup script in #7965, merged other changes in 81cc657