docs(lambda-extension): update distributed tracing headers warning;

[aztalbot]

Description

Standard distributed tracing headers are now excluded from the cache key since hashicorp/vault-lambda-extension#145 was merged.

So, I've updated the warning about distributed tracing headers to focus on nonstandard headers. Such headers may still cause issues without consumers being aware of them. I didn't call it out, but for example: New Relic adds a vendor-specific distributed tracing header for backwards compatibility that needs to be explicitly turned off.

TODO only if you're a HashiCorp employee

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