gh-122196: Clarify, reorder language for TestCase(object)

Lib/unittest/case.py

@@ -308,18 +308,20 @@ def __iter__(self):
 class TestCase(object):
     """A class whose instances are single test cases.
 
-    By default, the test code itself should be placed in a method named
-    'runTest'.
+    Test authors should subclass this TestCase class for their own tests.
+    Testing code may be placed in subclass method named 'runTest';
+    alternatively, invoke unittest.main() from the subclass's "__name__ guard"
+    section to run all methods defined in the test class. To achieve more
+    precise subclass test invocation see below (viz., constructor kwarg).
 
-    If the fixture may be used for many test cases, create as
-    many test methods as are needed. When instantiating such a TestCase
-    subclass, specify in the constructor arguments the name of the test method
-    that the instance is to execute.
-
-    Test authors should subclass TestCase for their own tests. Construction
-    and deconstruction of the test's environment ('fixture') can be
+    Construction and destruction of the tests' environment ('fixture') can be
     implemented by overriding the 'setUp' and 'tearDown' methods respectively.
 
+    If the fixture will be used for several distinct test cases, create as
+    many test methods as needed in the TestCase subclass, then specify a
+    method to execute by providing the method name in the 'methodName'
+    kwarg of the instance constructor.
+
     If it is necessary to override the __init__ method, the base class
     __init__ method must always be called. It is important that subclasses
     should not change the signature of their __init__ method, since instances
@@ -331,8 +333,8 @@ class TestCase(object):
         the instance's assertion methods fail; test methods raising this
         exception will be deemed to have 'failed' rather than 'errored'.
     * longMessage: determines whether long messages (including repr of
-        objects used in assert methods) will be printed on failure in *addition*
-        to any explicit message passed.
+        objects used in assert methods) will be printed on failure in
+        *addition* to any explicit message passed.
     * maxDiff: sets the maximum length of a diff in failure messages
         by assert methods using difflib. It is looked up as an instance
         attribute so can be configured by individual tests if required.