Improve CHANGELOG and add some comments

Ref: #5768

changelog/2270.bugfix.rst

@@ -1 +1,2 @@
-Fix ``self`` reference in function scoped fixtures that are in a plugin class
+Fixed ``self`` reference in function-scoped fixtures defined plugin classes: previously ``self``
+would be a reference to a *test* class, not the *plugin* class.

src/_pytest/fixtures.py

@@ -897,6 +897,8 @@ def resolve_fixture_function(fixturedef, request):
         # request.instance so that code working with "fixturedef" behaves
         # as expected.
         if request.instance is not None:
+            # handle the case where fixture is defined not in a test class, but some other class
+            # (for example a plugin class with a fixture), see #2270
             if hasattr(fixturefunc, "__self__") and not isinstance(
                 request.instance, fixturefunc.__self__.__class__
             ):

testing/python/fixtures.py

@@ -3946,6 +3946,9 @@ def test_2(fix):
         reprec.assertoutcome(passed=2)
 
     def test_class_fixture_self_instance(self, testdir):
+        """Check that plugin classes which implement fixtures receive the plugin instance
+        as self (see #2270).
+        """
         testdir.makeconftest(
             """
             import pytest