Clarify docs for add-headers and proxy-set-headers

[ryanmckaytx]

add format description

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): ref #1849

Special notes for your reviewer:
Will follow up with second PR for logging

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[k8s-reviewable]

This change is 

[ryanmckaytx]

CLA completed

[coveralls]

Coverage remained the same at 36.764% when pulling b873f1c on ryanmckaytx:master into fead908 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@ryanmckaytx thanks!