docs: Add context to `sender-cleanup` in Postfix `master.cf` (#3834)

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

target/postfix/master.cf

@@ -46,6 +46,8 @@ pickup         fifo  n       -       n       60      1       pickup
   -o content_filter=
   -o receive_override_options=no_header_body_checks
 
+# This relates to submission(s) services defined above:
+# https://www.postfix.org/BUILTIN_FILTER_README.html#mx_submission
 sender-cleanup unix  n       -       n       -       0       cleanup
   -o syslog_name=postfix/sender-cleanup
   -o header_checks=pcre:/etc/postfix/maps/sender_header_filter.pcre