Add NAMESERVER_TTL and associated documentation. (#398)

docs/_functions/domain/NAMESERVER_TTL.md

@@ -0,0 +1,19 @@
+---
+name: NAMESERVER_TTL
+parameters:
+  - ttl
+---
+
+TTL sets the TTL on the domain apex NS RRs defined by [NAMESERVER](#NAMESERVER).
+
+The value can be an integer or a string. See [TTL](#TTL) for examples.
+
+{% include startExample.html %}
+{% highlight js %}
+
+D('example.com', REGISTRAR, DnsProvider('R53'),
+  NAMESERVER_TTL('2d'),
+  NAMESERVER('ns')
+);
+{%endhighlight%}
+{% include endExample.html %}

docs/_providers/softlayer.md

@@ -40,10 +40,8 @@ For compatibility with the pre-generated NAMESERVER fields it's recommended to s
 
 {% highlight js %}
 D("example.tld", REG_NONE, DnsProvider(SOFTLAYER),
-    {"ns_ttl": "86400"},
+    NAMESERVER_TTL(86400),
 
     A("test","1.2.3.4")
 );
-{%endhighlight%}
-
-`ns_ttl` is a standard metadata field that applies to all providers.
+{%endhighlight%}

docs/examples.md

@@ -34,7 +34,7 @@ D('example.com', REG, DnsProvider('GCLOUD'),
 D('example.com', registrar,
     DefaultTTL('5m'), // Default for a domain
     A('@', '1.2.3.4', TTL('10m')), // individual record
-    {'ns_ttl': '600'} // On domain apex NS RRs
+    NAMESERVER_TTL('10m') // On domain apex NS RRs
 );
 
 {% endhighlight %}