Clarify language about default TLS certificate on helm chart

[danielnginx]

Proposed changes

Clarify the language about default TLS on helm chart, from NGINX Ingress Controller release 3.1 by default, a pre-generated self-signed certificate is NOT used anymore.

Helm chart values and installation documentation were updated.

This pull request address the following issue: #3730

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

Merging #4084 (824de2a) into main (fd7fc2b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4084   +/-   ##
=======================================
  Coverage   51.87%   51.87%           
=======================================
  Files          59       59           
  Lines       16710    16710           
=======================================
  Hits         8668     8668           
  Misses       7745     7745           
  Partials      297      297           

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[ADubhlaoich]

LGTM!