Add a doc example for simplifiable-if-expression

Refs #5953
Closes #5882

Author: Pierre-Sassoulas

doc/data/messages/s/simplifiable-if-expression/bad.py

@@ -0,0 +1,8 @@
+FLYING_THINGS = ["bird", "plane", "superman", "this example"]
+
+def is_flying_animal(an_object):
+    if isinstance(an_object, Animal) and an_object in FLYING_THINGS:  # [simplifiable-if-expression]
+        is_flying = True
+    else:
+        is_not_flying = True
+    return is_flying, is_not_flying

doc/data/messages/s/simplifiable-if-expression/good.py

@@ -1 +1,5 @@
-# This is a placeholder for correct code for this message.
+FLYING_THINGS = ["bird", "plane", "superman", "this example"]
+
+def is_flying_animal(an_object):
+    is_flying = isinstance(an_object, Animal) and an_object.name in FLYING_THINGS
+    return is_flying, not is_flying

doc/data/messages/s/simplifiable-if-expression/related.rst

@@ -0,0 +1 @@
+- `Simplifying an 'if' statement with bool() <https://stackoverflow.com/questions/49546992/>`_