Update executor and observable docs (#2594)

docs/source/guide/executors.md

@@ -30,7 +30,9 @@ To instantiate an `Executor`, provide a function which either:
 1. Inputs a `mitiq.QPROGRAM` and outputs a `mitiq.QuantumResult`.
 2. Inputs a sequence of `mitiq.QPROGRAM`s and outputs a sequence of `mitiq.QuantumResult`s.
 
-**The function must be [annotated](https://peps.python.org/pep-3107/) to tell Mitiq which type of `QuantumResult` it returns. Functions with no annotations are assumed to return `float`s.**
+```{warning}
+To avoid confusion and invalid results, the executor function must be [annotated](https://peps.python.org/pep-3107/) to tell Mitiq which type of `QuantumResult` it returns. Functions without annotations are assumed to return `float`s.
+```
 
 A `QPROGRAM` is "something which a quantum computer inputs" and a `QuantumResult` is "something which a quantum computer outputs." The latter is canonically a bitstring for real quantum hardware, but can be other objects for testing, e.g. a density matrix.
 

docs/source/guide/observables.md

@@ -128,8 +128,8 @@ obs.expectation(circuit, execute=mitiq_cirq.sample_bitstrings)
 
 In error mitigation techniques, you can provide an observable to specify the expectation value to mitigate.
 
-```{admonition} Note:
-When specifying an `Observable`, you must ensure that the return type of the executor function is `MeasurementResultLike` or `DensityMatrixLike`.
+```{warning}
+As note in the [executor documentation](./executors.md#the-input-function), the executor must be annotated with the appropriate type hinting for the return type. Additionally, when specifying an `Observable`, you must ensure that the return type of the executor function is `MeasurementResultLike` or `DensityMatrixLike`.
 ```
 
 ```{code-cell} ipython3