Improve fixture message

This isn't perfect, but it seems like a big improvement over the status quo.

Closes #3018.

Co-Authored-By: Stuart Cook <Zalathar@users.noreply.github.com>

hypothesis-python/src/_hypothesis_pytestplugin.py

@@ -44,6 +44,19 @@
     SEED_OPTION,
     EXPLAIN_OPTION,
 ]
+_FIXTURE_MSG = """Function-scoped fixture {0!r} used by {1!r}
+
+Function-scoped fixtures are not reset between examples generated by
+`@given(...)`, which is often surprising and can cause subtle test bugs.
+
+If you were expecting the fixture to run separately for each generated example,
+then unfortunately you will need to find a different way to achieve your goal
+(e.g. using a similar context manager instead of a fixture).
+
+If you are confident that your test will work correctly even though the
+fixture is not reset between generated examples, you can suppress this health
+check to assure Hypothesis that you understand what you are doing.
+"""
 
 
 class StoringReporter:
@@ -222,15 +235,6 @@ def pytest_runtest_call(item):
                 # Warn about function-scoped fixtures, excluding autouse fixtures because
                 # the advice is probably not actionable and the status quo seems OK...
                 # See https://github.com/HypothesisWorks/hypothesis/issues/377 for detail.
-                msg = (
-                    "%s uses the %r fixture, which is reset between function calls but not "
-                    "between test cases generated by `@given(...)`.  You can change it to "
-                    "a module- or session-scoped fixture if it is safe to reuse; if not "
-                    "we recommend using a context manager inside your test function.  See "
-                    "https://docs.pytest.org/en/latest/how-to/fixtures.html"
-                    "#scope-sharing-fixtures-across-classes-modules-packages-or-session "
-                    "for details on fixture scope."
-                )
                 argnames = None
                 for fx_defs in item._request._fixturemanager.getfixtureinfo(
                     node=item, func=item.function, cls=None
@@ -243,7 +247,7 @@ def pytest_runtest_call(item):
                             if active_fx.scope == "function":
                                 fail_health_check(
                                     settings,
-                                    msg % (item.nodeid, fx.argname),
+                                    _FIXTURE_MSG.format(fx.argname, item.nodeid),
                                     HealthCheck.function_scoped_fixture,
                                 )