Improve docs regarding plt.close().

Author: anntzer

doc/users/faq.rst

@@ -287,6 +287,7 @@ the desired format::
     import matplotlib.pyplot as plt
     plt.plot([1, 2, 3])
     plt.savefig('myfig.png')
+    plt.close()
 
 .. seealso::
 

lib/matplotlib/pyplot.py

@@ -1172,7 +1172,7 @@ def disconnect(cid: int) -> None:
 
 def close(fig: None | int | str | Figure | Literal["all"] = None) -> None:
     """
-    Close a figure window.
+    Close a figure window, and unregister it from pyplot.
 
     Parameters
     ----------
@@ -1185,6 +1185,14 @@ def close(fig: None | int | str | Figure | Literal["all"] = None) -> None:
         - ``str``: a figure name
         - 'all': all figures
 
+    Notes
+    -----
+    pyplot maintains a reference to figures created with `figure()`. When
+    work on the figure is completed, it should be closed, i.e. deregistered
+    from pyplot, to free its memory (see also :rc:figure.max_open_warning).
+    Closing a figure window created by `show()` automatically deregisters the
+    figure. For all other use cases, most prominently `savefig()` without
+    `show()`, the figure must be deregistered explicitly using `close()`.
     """
     if fig is None:
         manager = _pylab_helpers.Gcf.get_active()