Explain use of global thread-local variable in repr.

It's not completely obvious to a reader why we need a thread-local
"global" variable to track reference cycles in `__repr__` calls,
and the test does not currently contain one. This change adds a comment
explaining the need and also adds a non-attrs value in the reference
cycle of `test_infinite_recursion`.

src/attr/_make.py

@@ -1864,6 +1864,14 @@ def _add_eq(cls, attrs=None):
  return cls
 
 
+# Thread-local global to track attrs instances which are already being repr'd.
+# This is needed because there is no other (thread-safe) way to pass info
+# about the instances that are already being repr'd through the call stack
+# in order to ensure we don't perform infinite recursion.
+#
+# For instance, if an instance contains a dict which contains that instance,
+# we need to know that we're already repr'ing the outside instance from within
+# the dict's repr() call.
 _already_repring = threading.local()
 
 if HAS_F_STRINGS:

tests/test_dunders.py

@@ -364,8 +364,10 @@ class Cycle(object):
  cycle = attr.ib(default=None)
 
  cycle = Cycle()
- cycle.cycle = cycle
- assert "Cycle(value=7, cycle=...)" == repr(cycle)
+ # Ensure that the reference cycle passes through a non-attrs object.
+ # This demonstrates the need for a thread-local "global" ID tracker.
+ cycle.cycle = [cycle]
+ assert "Cycle(value=7, cycle=[...])" == repr(cycle)
 
  def test_underscores(self):
  """