Update documentation for rate limiting

[skomma]

limit-rate/limit-rate-after annotations must be used with proxy-buffering enabled, but no documentation about this exists.
So this PR adds the notes of these annotations.

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Hi @skomma. Thanks for your PR.

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[skomma]

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

I've just signed CLA.

[skomma]

/assign @ElvinEfendi

[aledbf]

/ok-to-test

[codecov-io]

Codecov Report

Merging #4749 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4749      +/-   ##
==========================================
+ Coverage    58.6%   58.76%   +0.15%     
==========================================
  Files          88       88              
  Lines        6743     6836      +93     
==========================================
+ Hits         3952     4017      +65     
- Misses       2355     2374      +19     
- Partials      436      445       +9

Impacted Files	Coverage Δ
internal/ingress/controller/store/backend_ssl.go	42.47% <0%> (-0.83%)	⬇️
internal/ingress/controller/controller.go	51.56% <0%> (+1.13%)	⬆️
internal/ingress/controller/store/store.go	60.12% <0%> (+1.48%)	⬆️
internal/watch/file_watcher.go	84.61% <0%> (+3.84%)	⬆️

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[aledbf]

/lgtm

[aledbf]

@skomma thanks!

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[APPROVALNOTIFIER] This PR is APPROVED

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

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