Rspamd: minor tweaks and follow-up for SPF, DKIM and DMARC symbols (#3923)

* move `policies_group.conf` to correct location

I originally assumed the file had to be placed into `scores.d`, but I
now know that `local.d` is actually correct.

* add configuration for composite symbols

See updates to #3690:

Additional Rspamd Symbols

Rspamd has so-called composite symbols that trigger when a condition
is met. Especially AUTH_NA and AUTH_NA_OR_FAIL will adjust the scores
of various lines in the table above. This needs to be taken into account.

* update CHANGELOG

target/rspamd/local.d/composites.conf

@@ -0,0 +1,18 @@
+# In addition to `policies_group.conf`, this file contains
+# symbols that are applied when certain other symbols are
+# applied (or not applied).
+#
+# We are especially interested in the `policy` field, because
+# there are cases in which `remove_weight` is undesirable.
+
+# When neither SPF, DKIM, nor DMARC are available, we want
+# to increase the base score so we apply at least greylisting.
+AUTH_NA {
+  score = 2.5;
+  policy = "leave";
+}
+
+AUTH_NA_OR_FAIL {
+  score = 1;
+  policy = "leave";
+}