Beats Doc Updates for 5.1

[dedemorton]

This is a meta issue to track Beats documentation work for 5.1.

If you have trouble clicking an item in the checklist to mark it as complete, open this comment in edit mode and change the source to [x]. Seems to be a bug in ticking the check boxes in this issue.

5.1:

• Document new processors:
  • add_cloud_metadata processor (see Document add_cloud_metadata processor #2791)
  • [DeDe]json_decode_files processor (see Document json_decode_fields processor #2923 and json_decode_fields processor  #2605)
• Document Metricbeat docker module
• Resolve any doc issues flagged for 5.1.
• [DeDe] Doc edits left over from 5.0:
  • Conditionals
  • HAProxy
  • New config options (shutdown_timeout, etc)

NOTE: Moved "future" list to the meta issue for 5.2.