Specify IGate meaning in the docs (#15462)

Author: Cryoris

qiskit/circuit/library/standard_gates/i.py

@@ -22,8 +22,8 @@
 class IGate(SingletonGate):
     r"""Identity gate.
 
-    Identity gate corresponds to a single-qubit gate wait cycle,
-    and should not be optimized or unrolled (it is an opaque gate).
+    This typically represents a single-qubit idle cycle.
+    For device-specific information, refer to the device's :class:`.Target`.
 
     Can be applied to a :class:`~qiskit.circuit.QuantumCircuit`
     with the :meth:`~qiskit.circuit.QuantumCircuit.i` and