Add target() example to docs

hypothesis-python/docs/details.rst

@@ -420,6 +420,19 @@ there is a natural metric like "floating-point error", "load factor" or
 
 .. autofunction:: hypothesis.target
 
+.. code-block:: python
+
+  from hypothesis import given, strategies as st, target
+
+
+  @given(st.floats(0, 1e100), st.floats(0, 1e100), st.floats(0, 1e100))
+  def test_associativity_with_target(a, b, c):
+      ab_c = (a + b) + c
+      a_bc = a + (b + c)
+      difference = abs(ab_c - a_bc)
+      target(difference)  # Without this, the test almost always passes
+      assert difference < 2.0
+
 We recommend that users also skim the papers introducing targeted PBT;
 from `ISSTA 2017 <http://proper.softlab.ntua.gr/papers/issta2017.pdf>`__
 and `ICST 2018 <http://proper.softlab.ntua.gr/papers/icst2018.pdf>`__.