Expose keyword length in the CSV and doc

[ph]

When adding new fields in the schema we can impose length limits on the string, the value of the limit is only exposed in the YAML and the generated templates.

As a producer of events, I think it would be appropriate to expose that value in the documentation and the CSV for easy reference and also reduce the risk of sending data that will be truncated.

[jamiehynds]

Closing as we haven't seen any requests for this since ECS 1.0.