docs(client): clarify private_key_jwt reserved-claim behavior

[nielskaspers]

Fixes #1914.

Clarifies that createPrivateKeyJwtAuth({ claims }) merges additional payload keys, but the SDK always sets the standard JWT claims (iss, sub, aud, exp, iat, jti) explicitly and they override any overlapping keys in claims.

Docs-only change; no runtime behavior changes.

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