Call them network upgrades rather than network updates to better indicate that they are DB upgrades rather than code/file upgrades.

Improve the contextual help for the Network Upgrade screen.

Props DrewAPicture
fixes #23540


git-svn-id: https://core.svn.wordpress.org/trunk@23619 1a063a9b-81f0-0310-95a4-ce76da25c4cd