Change 'Signup Complete' to more descriptive prompt

By prompting users to check their email to activate their accounts, it's hoped
that there will be less confusion when new members register for accounts.

Fixes #4962

Props themightymo, vegasgeek

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6994 cdf35c40-ae34-48e0-9cc9-0c9da1808c22