Document and increase the maxpeers setting.

To merge *after* my [error message pull-request](https://github.com/freenet/fred/pull/283), because I pushed before realizing that I had worked on the wrong branch. Please do not rebase published history.
This is a bit too theoretical. In practice, nodes often do not connect (they actively reject us, have no free slots, are behind NAT or are for some other reason unable to connect).
One of my nodes, which is on a 1 Gb link with direct IPv4 and IPv6 internet connectivity (i.e. more or less the ideal case) is having a hard time connecting to more than 90 peers at this moment.