Add documentation to Pipeline.swift

Author: cherylEnkidu

Firestore/Swift/Source/SwiftAPI/Pipeline/Expr.swift

@@ -8,7 +8,7 @@
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF Sendable KIND, either express or implied.
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.