[exim] docs nit

http://exim.org/exim-html-4.63/doc/html/spec_html/ch11.html#SECTexpvar

Is $domain already lower-cased? The docs don't say one way or the other.
What about $original_domain and $parent_domain?
$sender_address?

Note that $local_part is specified for this.


Thanks,
Jeremy