Add annotation doc on proxy buffer size

[qiujian16]

What this PR does / why we need it:
This is to add a doc about proxy buffer size annotation

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #
fix #2527

Special notes for your reviewer:

[codecov-io]

Codecov Report

Merging #2625 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2625      +/-   ##
==========================================
+ Coverage   40.88%   40.94%   +0.05%     
==========================================
  Files          75       75              
  Lines        5129     5129              
==========================================
+ Hits         2097     2100       +3     
+ Misses       2753     2751       -2     
+ Partials      279      278       -1

Impacted Files	Coverage Δ
cmd/nginx/main.go	25.37% <0%> (+2.23%)	⬆️

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[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, qiujian16

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[aledbf]

@qiujian16 thanks!