NEW MACROS: DOMAIN_ELSEWHERE and DOMAIN_ELSEWHERE_AUTO (#1237)

* NEW MACROS: DOMAIN_ELSEWHERE and DOMAIN_ELSEWHERE_AUTO
* Finish docs

docs/_functions/global/DOMAIN_ELSEWHERE.md

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+---
+name: DOMAIN_ELSEWHERE
+parameters:
+  - registrar
+  - list of nameserver names
+---
+
+`DOMAIN_ELSEWHERE()` is a helper macro that lets you easily indicate that
+a domain's zones are managed elsewhere. That is, it permits you easily delegate
+a domain to a hard-coded list of DNS servers.
+
+`DOMAIN_ELSEWHERE` is useful when you control a domain's registrar but not the
+DNS servers. For example, suppose you own a domain but the DNS servers are run
+by someone else, perhaps a SaaS product you've subscribed to or a DNS server
+that is run by your brother-in-law who doesn't trust you with the API keys that
+would let you maintain the domain using DNSControl. You need an easy way to
+point (delegate) the domain at a specific list of DNS servers.
+
+For example these two statements are equivalent:
+
+```
+DOMAIN_ELSEWHERE("example.com", REG_NAMEDOTCOM, ["ns1.foo.com", "ns2.foo.com"]);
+
+# ...is equivalent to...
+
+D("example.com", REG_NAMEDOTCOM,
+    NO_PURGE,
+    NAMESERVER("ns1.foo.com"),
+    NAMESERVER("ns2.foo.com")
+);
+```
+
+NOTE: The `NO_PURGE` is used out of abundance of caution but since no
+`DnsProvider()` statements exist, no updates would be performed.

docs/_functions/global/DOMAIN_ELSEWHERE_AUTO.md

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+---
+name: DOMAIN_ELSEWHERE_AUTO
+parameters:
+  - registrar
+  - list of Dns Providers
+---
+
+`DOMAIN_ELSEWHERE_AUTO()` is similar to `DOMAIN_ELSEWHERE()` but instead of
+a hardcoded list of nameservers, a DnsProvider() is queried.
+
+`DOMAIN_ELSEWHERE_AUTO` is useful when you control a domain's registrar but the
+DNS zones are managed by another system. Luckily you have enough access to that
+other system that you can query it to determine the zone's nameservers.
+
+For example, suppose you own a domain but the DNS servers for it are in Azure.
+Further suppose that something in Azure maintains the zones (automatic or
+human). Azure picks the nameservers for the domains automatically, and that
+list may change occasionally.  `DOMAIN_ELSEWHERE_AUTO` allows you to easily
+query Azure to determine the domain's delegations so that you do not need to
+hard-code them in your dnsconfig.js file.
+
+For example these two statements are equivalent:
+
+```
+DOMAIN_ELSEWHERE_AUTO("example.com", REG_NAMEDOTCOM, DSP_AZURE);
+
+# ...is equivalent to...
+
+D("example.com", REG_NAMEDOTCOM,
+    NO_PURGE,
+    DnsProvider(DSP_AZURE)
+);
+```
+
+NOTE: The `NO_PURGE` is used to prevent DNSControl from changing the records.