Add docstring for cirq.TRANSFORMER public object (#5149)

Adds docstring for `cirq.TRANSFORMER` public object. The api reference automatically creates a reference page for the public object, and currently inherits the docstring from the `Protocol` base class, which is irrelevant and the PR fixes it.

cirq-core/cirq/transformers/transformer_api.py

@@ -229,6 +229,37 @@ class TransformerContext:
 
 
 class TRANSFORMER(Protocol):
+    """Protocol class defining the Transformer API for circuit transformers in Cirq.
+
+    Any callable that satisfies the `cirq.TRANSFORMER` contract, i.e. takes a `cirq.AbstractCircuit`
+    and `cirq.TransformerContext` and returns a transformed `cirq.AbstractCircuit`, is a valid
+    transformer in Cirq.
+
+    Note that transformers can also accept additional arguments as `**kwargs`, with default values
+    specified for each keyword argument. A transformer could be a function, for example:
+
+    >>> def convert_to_cz(
+    >>>    circuit: cirq.AbstractCircuit,
+    >>>    *,
+    >>>    context: Optional[cirq.TransformerContext] = None,
+    >>>    atol: float = 1e-8,
+    >>> ) -> cirq.Circuit:
+    >>>     ...
+
+    Or it could be a class that implements `__call__` with the same API, for example:
+
+    >>> class ConvertToSqrtISwaps:
+    >>>    def __init__(self):
+    >>>        ...
+    >>>    def __call__(
+    >>>        self,
+    >>>        circuit: cirq.AbstractCircuit,
+    >>>        *,
+    >>>        context: Optional[cirq.TransformerContext] = None,
+    >>>    ) -> cirq.AbstractCircuit:
+    >>>        ...
+    """
+
     def __call__(
         self, circuit: 'cirq.AbstractCircuit', *, context: Optional[TransformerContext] = None
     ) -> 'cirq.AbstractCircuit':