[Docs] Missing description for 'xpack.encrypted_saved_objects'

[peterpramb]

Kibana version:
7.5.1

Describe the bug:
Kibana produces warnings on startup in the log if xpack.encrypted_saved_objects.encryptionKey is not set.

While it provides instructions on what to do, I was not able to locate any documentation for it nor what this is about at all.

Provide logs and/or server output (if relevant):

{
    "@timestamp": "2020-01-21T12:31:00Z",
    "message": "Generating a random key for xpack.encrypted_saved_objects.encryptionKey. To be able to decrypt encrypted saved objects attributes after restart, please set xpack.encrypted_saved_objects.encryptionKey in kibana.yml",
    "pid": 1529,
    "tags": [
        "warning",
        "encrypted_saved_objects"
    ],
    "type": "log"
}

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[peterschretlen]

I'll do an initial pass of the doc, there are a few others related to alerting and actions that need to be documented as well.

[kobelb]

Thanks @peterschretlen, lemme know if you could use any help from @elastic/kibana-security, or if you'd like us to take any of the documentation on.