Add docs around how does the AWS LB controller work with legacy cloud provider

[kolorful]

Add docs around how does the AWS LB controller work with legacy cloud provider, which was mentioned in #1978 (comment)

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[k8s-ci-robot]

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[kishorj]

@kolorful, thanks for the contribution.

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kishorj, kolorful

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[codecov-commenter]

Codecov Report

Merging #1988 (5b0f4be) into main (502a5d3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1988   +/-   ##
=======================================
  Coverage   48.89%   48.89%           
=======================================
  Files         130      130           
  Lines        6991     6991           
=======================================
  Hits         3418     3418           
  Misses       3296     3296           
  Partials      277      277           

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[kishorj]

/retest