doc log-format: add variables about ingress

[vgramer]

What this PR does / why we need it:
Add the missing ingress information variables to the log-format doc.

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[codecov-io]

Codecov Report

Merging #2816 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2816      +/-   ##
==========================================
+ Coverage   47.34%   47.47%   +0.12%     
==========================================
  Files          75       75              
  Lines        5413     5413              
==========================================
+ Hits         2563     2570       +7     
+ Misses       2525     2519       -6     
+ Partials      325      324       -1

Impacted Files	Coverage Δ
cmd/nginx/main.go	11.18% <0%> (+4.6%)	⬆️

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[antoineco]

Thanks @vgramer, please sign the CLA.

[vgramer]

@antoineco done :)

[antoineco]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: antoineco, vgramer

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