Add integration test for SOA (#1147)

* Add integration test for SOA

* Add docs for SOA record

* MAINT: Rename SoaInfo to SoaDefaults.

gorename -v -from '"github.com/StackExchange/dnscontrol/v3/providers/bind".SoaInfo' -to SoaDefaults

docs/_functions/domain/SOA.md

@@ -0,0 +1,30 @@
+---
+name: SOA
+parameters:
+  - name
+  - ns
+  - mbox
+  - refresh
+  - retry
+  - expire
+  - minttl
+  - modifiers...
+---
+
+`SOA` adds an `SOA` record to a domain. The name should be `@`.  ns and mbox are strings. The other fields are unsigned 32-bit ints.
+
+{% include startExample.html %}
+{% highlight js %}
+
+D("example.com", REG_THIRDPARTY, DnsProvider("DNS_BIND"),
+  SOA("@", "ns3.example.org.", "hostmaster.example.org.", 3600, 600, 604800, 1440),
+);
+
+{%endhighlight%}
+{% include endExample.html %}
+
+
+## Notes:
+
+* The serial number is managed automatically.  It isn't even a field in `SOA()`.
+* Most providers automatically generate SOA records.  They will ignore any `SOA()` statements.