[exim] gotcha: chunking and predata

In Exim 4.88 documentation:

> If CHUNKING was advertised and a BDAT command sequence is received, the
> acl_smtp_predata ACL is not run.

Unexpected. Why?
CHUNKING is advertised by default,
for example Gmail issues BDAT commands.

I think this at least deserves a mention in NewStuff.