K4 docs: Add a note on dynamic mappings

[tbragin]

We require that Elasticsearch "dynamic mapping" on fields is enabled (which is the default):
http://www.elastic.co/guide/en/elasticsearch/guide/current/dynamic-mapping.html

If not, the user will manually need to provide mappings for new fields to correct types for Kibana to correctly use them in visualizations.

In addition, dynamic mapping is required for ongoing management of the .kibana index.

[maharg101]

Could you include the relevant mapping in the documentation please.

[tbragin]

In addition to clarifying that dynamic indexing needs to be enabled for .kibana index on an ongoing basis (which is the default globally), we should document the process for setting dynamic indexing to true just for the .kibana index, if someone chooses to override the global setting. Something like this, maybe:

Assumptions:
* .kibana index does not exist yet
* elasticsearch.yml has the following setting: "index.mapper.dynamic": false

1. Start elasticsearch

2. Create the .kibana index with the dynamic mapping setting set to true:
PUT .kibana
{
  "index.mapper.dynamic": true
}

3. Start kibana, access the web UI - you should not get any error messages related to dynamic mapping. 

[nellicus]

+1

[palecur]

Added to docs in 46ee3ab for PR#3813.