add note about x86_64 to forwarder documentation

[cswatt]

What does this PR do?

Adds a note to documentation about supported architecture for forwarder v4.1.0+.

see #903 (comment)

Motivation

Testing Guidelines

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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• This PR contains breaking changes that are documented in the description
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