Clarify docs.

TODO

@@ -1,3 +1,8 @@
+Don't match dynamic ptr in bestguess.
+
+When content filtering is not installed, reject BLACKLISTed MFROM
+immediately.  There is no use waiting until EOM.
+
 Configuration is problematic when handling incoming, but not outgoing mail.
 The problem comes when alice@example.com sends mail to bill@example.com,
 and we are the MX for example.com, but alice is sending from some other
@@ -7,6 +12,9 @@ CBV, we get "fraudulent MX", because the MX is ourself!  So we need to
 avoid doing CBV on such domains.  Currently, we try to make sure the SPF
 policies don't do CBV.
 
+We now don't check internal domains for incoming mail if there is an
+SPF record.
+
 On the other hand, if alice is sending internally, or with SMTP AUTH, she
 *does* need the domain to be in internal_domains.  The solution to that 
 is to use the new SMTP AUTH access configuration to specify which domains