Move documentation contents from reference.rst to docstrings

It's better to have the documentation in one place, instead
of having some in the docstring and some additional
information added to the reference documentation in
`reference.rst`.

doc/en/reference/reference.rst

@@ -363,17 +363,6 @@ capsys
 .. autofunction:: _pytest.capture.capsys()
     :no-auto-options:
 
-    Returns an instance of :class:`CaptureFixture[str] <pytest.CaptureFixture>`.
-
-    Example:
-
-    .. code-block:: python
-
-        def test_output(capsys):
-            print("hello")
-            captured = capsys.readouterr()
-            assert captured.out == "hello\n"
-
 .. autoclass:: pytest.CaptureFixture()
     :members:
 
@@ -388,18 +377,6 @@ capsysbinary
 .. autofunction:: _pytest.capture.capsysbinary()
     :no-auto-options:
 
-    Returns an instance of :class:`CaptureFixture[bytes] <pytest.CaptureFixture>`.
-
-    Example:
-
-    .. code-block:: python
-
-        def test_output(capsysbinary):
-            print("hello")
-            captured = capsysbinary.readouterr()
-            assert captured.out == b"hello\n"
-
-
 .. fixture:: capfd
 
 capfd
@@ -410,18 +387,6 @@ capfd
 .. autofunction:: _pytest.capture.capfd()
     :no-auto-options:
 
-    Returns an instance of :class:`CaptureFixture[str] <pytest.CaptureFixture>`.
-
-    Example:
-
-    .. code-block:: python
-
-        def test_system_echo(capfd):
-            os.system('echo "hello"')
-            captured = capfd.readouterr()
-            assert captured.out == "hello\n"
-
-
 .. fixture:: capfdbinary
 
 capfdbinary
@@ -432,17 +397,6 @@ capfdbinary
 .. autofunction:: _pytest.capture.capfdbinary()
     :no-auto-options:
 
-    Returns an instance of :class:`CaptureFixture[bytes] <pytest.CaptureFixture>`.
-
-    Example:
-
-    .. code-block:: python
-
-        def test_system_echo(capfdbinary):
-            os.system('echo "hello"')
-            captured = capfdbinary.readouterr()
-            assert captured.out == b"hello\n"
-
 
 .. fixture:: doctest_namespace
 
@@ -453,16 +407,6 @@ doctest_namespace
 
 .. autofunction:: _pytest.doctest.doctest_namespace()
 
-    Usually this fixture is used in conjunction with another ``autouse`` fixture:
-
-    .. code-block:: python
-
-        @pytest.fixture(autouse=True)
-        def add_np(doctest_namespace):
-            doctest_namespace["np"] = numpy
-
-    For more details: :ref:`doctest_namespace`.
-
 
 .. fixture:: request
 
@@ -600,12 +544,6 @@ recwarn
 .. autoclass:: pytest.WarningsRecorder()
     :members:
 
-Each recorded warning is an instance of :class:`warnings.WarningMessage`.
-
-.. note::
-    ``DeprecationWarning`` and ``PendingDeprecationWarning`` are treated
-    differently; see :ref:`ensuring_function_triggers`.
-
 
 .. fixture:: tmp_path