Address error logging more clearly

[starchy]

The guide as written does not error logging and will record IPs for error logs for all users. This might not be what users or iplementers expect, and AFAICT this isn't really indicated until a single-line notice in section 11.

Note also the alternate logrotate config for nginx in issue #5 improves this slightly since it will discard IPs logged with errors after 7 (or 10) days.