panw: improve package doc text

[efd6]

What does this PR do?

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

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How to test this PR locally

Related issues

• Relates [panw] sync with beats module #2758.

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Build stats

• Start Time: 2022-03-10T01:29:56.288+0000

• Duration: 20 min 44 sec

Test stats 🧪

Test	Results
Failed	0
Passed	31
Skipped	0
Total	31

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