docs: Update iat exception docs

[pachewise]

Updating documentation given which exception is thrown when. Source:

pyjwt/jwt/api_jwt.py

Lines 268 to 279 in d7710f7

	def _validate_iat(
	self,
	payload: dict[str, Any],
	now: float,
	leeway: float,
	) -> None:
	try:
	iat = int(payload["iat"])
	except ValueError:
	raise InvalidIssuedAtError("Issued At claim (iat) must be an integer.")
	if iat > (now + leeway):
	raise ImmatureSignatureError("The token is not yet valid (iat)")