Fix docstring lib/matplotlib/pyplot.py and related lib/matplotlib/__init__.py (matplotlib#31168)

* DOC: Remove inaccurate claim that switch_backend closes all figures

The switch_backend docstring claimed that all open figures would be
closed via plt.close('all') when switching to a different backend,
but the implementation never actually does this.

* DOC: Remove same inaccurate close-all claim from matplotlib.use docstring

The matplotlib.use docstring also claimed figures would be closed when
switching backends, which is equally incorrect.

Author: alberti42

lib/matplotlib/__init__.py

@@ -1192,8 +1192,7 @@ def use(backend, *, force=True):
     Select the backend used for rendering and GUI integration.
 
     If pyplot is already imported, `~matplotlib.pyplot.switch_backend` is used
-    and if the new backend is different than the current backend, all Figures
-    will be closed.
+    to switch the backend.
 
     Parameters
     ----------

lib/matplotlib/pyplot.py

@@ -398,9 +398,6 @@ def switch_backend(newbackend: str) -> None:
     another interactive backend has started.  Switching to and from
     non-interactive backends is always possible.
 
-    If the new backend is different than the current backend then all open
-    Figures will be closed via ``plt.close('all')``.
-
     Parameters
     ----------
     newbackend : str