Improve "Certificate for subdomains excluding apex domain" section

* Remove "I want a" from title :)
* The ACME severs is unspecific.
* `pki_acme` is unrelated to this section.
* Comments after YAML variables might case problems in certain YAML
  modes and should thus be avoided.
* Use the more specific term "apex domain" as already suggested by
  @drybjed in: debops#82 (comment)

Related to: debops#82
Wanted confirmation from: @ser, @drybjed

docs/acme-integration.rst

@@ -155,31 +155,32 @@ configuration active, it will check for validity of the signed certificate, and
 about a month before the expiration date it will try to renew the certificate
 automatically.
 
-I want a certificate for subdomains but domain
-----------------------------------------------
+Certificate for subdomains excluding apex domain
+------------------------------------------------
 
 Yes, it's possible :-) Please consult the example and create your own similar
-configuration. In the example we create a certificate for ``logs.example.com``
-and ``mon.example.com`` subdoimains, without creating cert for ``example.com``
-domain itself. Please notice that PKI realm does not contain your full domain
-name, it's crucial.
+configuration. In the example we create a certificate for the ``logs.example.com``
+and ``mon.example.com`` subdomains, which does not include the ``example.com``
+apex domain. Please notice that the PKI realm does not contain your full domain
+name. This is crucial.
 
 .. code-block:: yaml
 
-    pki_acme: True
     pki_realms:
-      - name: 'example' # do not include full domain name here!
+        # Do not include the full domain name here!
+      - name: 'example'
         acme: True
         acme_default_subdomains: []
         acme_subject: [ 'cn=logs.example.com' ]
         acme_domains: [ 'logs.example.com', 'mon.example.com' ]
         domains: [ 'logs.example.com', 'mon.example.com' ]
-        #acme_ca: 'le-staging'
+        # acme_ca: 'le-staging'
 
-For testing it's strongly advised to uncomment ``acme_ca`` with ``le-staging`` to 
-use testing ACME servers. It does not create a real cert, but allows you to avoid
-problems with usual ACME servers rate limits. When you are sure that everything works
-correctly, comment the staging environment back.
+For testing it's strongly advised to uncomment ``acme_ca`` with ``le-staging``
+to use the staging environment of Let's Encrypt. It does not create a trusted
+certificate and allows you to avoid problems with the rate limits in the
+production environment. When you are sure that everything works correctly,
+comment the staging environment back.
 
 ACME configuration variables
 ----------------------------