docs: Add release note and docs to credentials refactoring

[orbaps]

Changes

Changes
Document usage of FUNC_USERNAME, FUNC_PASSWORD, and FUNC_TOKEN environment variables in func deploy help text
Document usage of FUNC_USERNAME, FUNC_PASSWORD, and FUNC_TOKEN environment variables in func build help text
Update generated reference documentation for deploy and build commands
/kind documentation

Fixes #3299

Release Note

The FUNC_USERNAME and FUNC_PASSWORD environment variables (and FUNC_TOKEN) now work for registry authentication in func deploy and func build commands across different pushers.
Docs

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[knative-prow]

Welcome @orbaps! It looks like this is your first PR to knative/func 🎉

[knative-prow]

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