Networks and Sites: Clarify the recommendation to remove the `www` prefix from...

Networks and Sites: Clarify the recommendation to remove the `www` prefix from the site domain before creating a network.

The `siteurl` option reference was not quite clear, and the string actually refers to the domain name, not the full URL.

Props kharisblank, tobifjellner, ramiy.
Fixes #49214.
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