Explain Site and User count update frequency in Network Admin contextual help

[jeremyfelt]

While adding a bunch of sites to a multisite install today, I noticed for the first time that the counts (option blog_count) for sites and users in the Right Now box don't update immediately. I was confused, but then saw 15170 which moved the count update to a twicedaily cron for scalability reasons.

It may be helpful to include this information in the contextual help for others who will find themselves confused. Attached patch is an attempt at that.