Move DNS cache docs to system configuration docs

When these docs were moved they should have been moved to the system
configuration docs. This commit does that, and also fixes a missing
heading that broke the docs build.

docs/reference/setup/important-settings.asciidoc

@@ -201,22 +201,3 @@ the Elasticsearch process. If you wish to configure a heap dump path, you should
 modify the entry `#-XX:HeapDumpPath=/heap/dump/path` in
 <<jvm-options,`jvm.options`>> to remove the comment marker `#` and to specify an
 actual path.
-
-[float]
-[[networkaddress-cache-ttl]]
-
-Elasticsearch runs with a security manager in place. With a security manager in
-place, the JVM defaults to caching positive hostname resolutions
-indefinitely. If your Elasticsearch nodes rely on DNS in an environment where
-DNS resolutions vary with time (e.g., for node-to-node discovery) then you might
-want to modify the default JVM behavior.  This can be modified by adding
-http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.ttl=<timeout>`]
-to your
-http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java
-security policy]. Any hosts that fail to resolve will be logged. Note also that
-with the Java security manager in place, the JVM defaults to caching negative
-hostname resolutions for ten seconds. This can be modified by adding
-http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.negative.ttl=<timeout>`]
-to your
-http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java
-security policy].

docs/reference/setup/sysconfig.asciidoc

@@ -41,5 +41,4 @@ include::sysconfig/virtual-memory.asciidoc[]
 
 include::sysconfig/threads.asciidoc[]
 
-
-
+include::sysconfig/dns-cache.asciidoc[]

docs/reference/setup/sysconfig/dns-cache.asciidoc

@@ -0,0 +1,18 @@
+[[networkaddress-cache-ttl]]
+=== DNS cache settings
+
+Elasticsearch runs with a security manager in place. With a security manager in
+place, the JVM defaults to caching positive hostname resolutions
+indefinitely. If your Elasticsearch nodes rely on DNS in an environment where
+DNS resolutions vary with time (e.g., for node-to-node discovery) then you might
+want to modify the default JVM behavior.  This can be modified by adding
+http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.ttl=<timeout>`]
+to your
+http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java
+security policy]. Any hosts that fail to resolve will be logged. Note also that
+with the Java security manager in place, the JVM defaults to caching negative
+hostname resolutions for ten seconds. This can be modified by adding
+http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.negative.ttl=<timeout>`]
+to your
+http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java
+security policy].