Include updated name/description in email notification after a group is updated.

When updating a group's details, there's a checkbox that asks "Notify group
members?". But the notification that goes out does not currently contain any
information about what was changed. The current changeset adds this information
to the email text.

Fixes #5879.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9226 cdf35c40-ae34-48e0-9cc9-0c9da1808c22