Subdomain multisite setup

Dear all,
I’m not an expert so I’d like to some help before I do something wrong!
I currently have a website (in one server) set up in the following way:

• an old fashioned html website with an associated domain name (let’s call it mainsite.com)
• a mono-site wordpress install with another domain name (let’s call it blogsite.com), for which the files are located in a subfolder of public_html called “public_html/blogsite.com/”

Now, I would like to “convert” the html website into a wordpress site and thus would transform the current install of wordpress into a multisite install.
From what I read, it can only be a subdomain based install since there is already an active wordpress install.
However, in the https://codex.wordpress.org/Before_You_Create_A_Network page, it is stated

If you want to use a sub-domain install, you must install WordPress in the root of your webpath (i.e. domain.com) however it does not need to be installed in the root (i.e. /public_html/) if you choose to run WordPress from it’s own directory.

and I’m not sure what it means exactly?
Is there anything else I should be careful to in order to perform this multisite install?