Update docs for more clarification on install plugins from chart version 0.12.0

[melissapalmer]

What this PR does / why we need it:

Update docs for more clarification on install plugins from chart version 0.12.0

Which issue this PR fixes

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

• relates to [stable/sonarqube] overiding image.tag=7.4-community does not install default sonarqube plugins #10195

Special notes for your reviewer:

Update docs to avoid confusion as on #10195 and PR #10473

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Variables are documented in the README.md

[k8s-ci-robot]

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