Docs: Minorly improve the hook doc summary for the...

Docs: Minorly improve the hook doc summary for the `pre_network_site_new_created_user` action, introduced in [35786].

See #33631. See #35986.

Built from https://develop.svn.wordpress.org/trunk@36826


git-svn-id: https://core.svn.wordpress.org/trunk@36793 1a063a9b-81f0-0310-95a4-ce76da25c4cd