Document {http.vars.trace_id} being set using tracing

[trajano]

This will allow people to add

header >x-request-id {http.vars.trace_id}

To the headers if they need it.

[francislavoie]

Makes sense, can be mentioned in here somewhere https://github.com/caddyserver/website/blob/master/src/docs/markdown/caddyfile/directives/tracing.md