increase index.max_docvalue_fields_search from 300 to 400

[markdboyd]

Changes proposed in this pull request:

Necessary to fix observed errors in Logsearch:

Trying to retrieve too many docvalue_fields. Must be less than or equal to: [300] but was [373]. This limit can be set by changing the [index.max_docvalue_fields_search] index level setting

security considerations

Should be no security impact, just increase limit to address error