build, deploy, inbound, dmarc, backup, e2e: cite the source of what they rely on (assumptions part C)

Comments only, no behaviour changed. Each main.cf/master.cf parameter the
build sets, the wrapper's and the reload hook's reliance on Postfix 3.7.11
(postfix-script, master.5, postconf.5, cleanup_init.c, smtpd_peer.c,
README_FILES), supervisor 4.2.5 (process.py, options.py, events.rst),
logrotate 3.21.0, OpenDKIM's config, Cyrus SASL's sasldb, rspamd 3.11.1
(milter_headers.lua, multimap, the map watcher), Docker Compose, CoreDNS
and certbot now say where that is written. Measurements already there stay
measurements ("date not recorded" where none was); hand-composed fixtures
say so.

Contradictions left for their own commits: C-P31 cleanup's size limit,
C-P42 master outside supervisor's signal, C-V5/V6 startsecs, C-V10,
C-S3/Q3 sasldb written in place, C-Q4/A1 certbot's symlinks, C-R7 rspamd
skips local clients but the signature routine does not, C-P8, C-R4, C-R12,
C-M7, C-E2, C-K8/K9, C-D1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/nginx/docker-compose.nginx.yml

@@ -8,7 +8,9 @@
 # takes over 80/443 and proxies to the panel over the compose network
 # instead). Merge it with the base file — run this from the deploy/ directory
 # (Compose resolves every relative path in both files against the directory of
-# the FIRST -f file, i.e. deploy/, which is why paths below are ./nginx/...):
+# the FIRST -f file, i.e. deploy/, which is why paths below are ./nginx/... -
+# docs.docker.com/compose/how-tos/multiple-compose-files/merge/: "Paths are
+# evaluated relative to the base file"):
 #
 #   docker compose -f docker-compose.yml -f nginx/docker-compose.nginx.yml up -d
 #
@@ -54,4 +56,7 @@ services:
       - ./nginx/certbot-www:/var/www/certbot
     # Renewal twice a day is certbot's own recommended cadence; it no-ops
     # until a certificate is within its renewal window.
+    # (eff-certbot.readthedocs.io, User Guide, "Automated Renewals": the
+    # sample cron runs "certbot renew" at 0 and 12 h; renew acts only on a
+    # certificate near expiry.)
     entrypoint: sh -c 'trap exit TERM; while :; do certbot renew; sleep 12h & wait $${!}; done'

deploy/nginx/nginx.conf.example

@@ -17,6 +17,8 @@ server {
 
 server {
 	listen 443 ssl;
+	# nginx.org/en/docs/http/ngx_http_v2_module.html, "http2": "This directive
+	# appeared in version 1.25.1" (the fragment runs nginx:1.27).
 	http2 on;
 	server_name mail.example.com;