Rephrase explaining how hosts groups work

Rephrase how to be added to a group

And add some more cases.

we can make this a different and better policy if we want, but now
we have something to start from.

i'm not sure what the best simple phrase is, but 'shared irc host'
might be better

it's basically the same process as other sysadmin changes

they were producing broken links of the form
https://help.torproject.org/tsa/doc/accounts/|https://db.torproject.org/login

Might resolve ticket 11308