MS: Update "Add New Site" address description text for accuracy.

Add hyphens to the list of allowed characters and add the `description` class to helper text paragraph tag.

Props thomaswm for the initial patch.
Fixes #26388.

Built from https://develop.svn.wordpress.org/trunk@35893


git-svn-id: https://core.svn.wordpress.org/trunk@35857 1a063a9b-81f0-0310-95a4-ce76da25c4cd