Add example of qasm2.CustomInstruction (#13167)

We didn't have an explicit usage example of this in the documentation
before, and several people seem to have been confused by it.

(cherry picked from commit f091cf2)

qiskit/qasm2/parse.py

@@ -89,6 +89,35 @@ class CustomInstruction:
     There is a final ``builtin`` field.  This is optional, and if set true will cause the
     instruction to be defined and available within the parsing, even if there is no definition in
     any included OpenQASM 2 file.
+
+    Examples:
+
+        Instruct the importer to use Qiskit's :class:`.ECRGate` and :class:`.RZXGate` objects to
+        interpret ``gate`` statements that are known to have been created from those same objects
+        during OpenQASM 2 export::
+
+            from qiskit import qasm2
+            from qiskit.circuit import QuantumCircuit, library
+
+            qc = QuantumCircuit(2)
+            qc.ecr(0, 1)
+            qc.rzx(0.3, 0, 1)
+            qc.rzx(0.7, 1, 0)
+            qc.rzx(1.5, 0, 1)
+            qc.ecr(1, 0)
+
+            # This output string includes `gate ecr q0, q1 { ... }` and `gate rzx(p) q0, q1 { ... }`
+            # statements, since `ecr` and `rzx` are neither built-in gates nor in ``qelib1.inc``.
+            dumped = qasm2.dumps(qc)
+
+            # Tell the importer how to interpret the `gate` statements, which we know are safe
+            # because we controlled the input OpenQASM 2 source.
+            custom = [
+                qasm2.CustomInstruction("ecr", 0, 2, library.ECRGate),
+                qasm2.CustomInstruction("rzx", 1, 2, library.RZXGate),
+            ]
+
+            loaded = qasm2.loads(dumped, custom_instructions=custom)
     """
 
     name: str