[exim] log_selector and the reject log

Hello, I think section 53.15 should be explicit that the log_selector
only applies to the main log. There are some log messages, namely
those from synprot_error in smtp_in.c, that go *both* to main and
reject log. It depends on log_selector if it in fact shows up in the
main log, but there is not such filtering logic for the reject log;
these log lines always show up there.

The behavior itself is somewhat counterintuitive to me, but my
intuition is weird at times, so I will let that pass. But it should
be documented to avoid confusing people who want to minimize log
volume.

--
Ian