[3.15] gh-151623: Improve curses documentation and docstrings (GH-151625) (GH-151628)

Fix errors and clarify the curses, curses.panel and curses.ascii docs
against X/Open Curses and ncurses, and sync the affected docstrings.
(cherry picked from commit 65afcdd)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: miss-islington

Doc/library/curses.ascii.rst

@@ -114,7 +114,7 @@ C library:
 
 .. function:: isblank(c)
 
-   Checks for an ASCII whitespace character; space or horizontal tab.
+   Checks for an ASCII blank character; space or horizontal tab.
 
 
 .. function:: iscntrl(c)
@@ -168,7 +168,8 @@ C library:
 
 .. function:: isctrl(c)
 
-   Checks for an ASCII control character (ordinal values 0 to 31).
+   Checks for an ASCII control character (ordinal values 0 to 31).  Unlike
+   :func:`iscntrl`, this does not include the delete character (0x7f).
 
 
 .. function:: ismeta(c)

Doc/library/curses.panel.rst

@@ -26,7 +26,8 @@ The module :mod:`!curses.panel` defines the following functions:
 
 .. function:: new_panel(win)
 
-   Returns a panel object, associating it with the given window *win*. Be aware
+   Returns a panel object, associating it with the given window *win* and
+   placing the new panel on top of the panel stack.  Be aware
    that you need to keep the returned panel object referenced explicitly.  If you
    don't, the panel object is garbage collected and removed from the panel stack.
 
@@ -99,7 +100,8 @@ Panel objects have the following methods:
 
 .. method:: Panel.show()
 
-   Display the panel (which might have been hidden).
+   Display the panel (which might have been hidden), placing it on top of
+   the panel stack.
 
 
 .. method:: Panel.top()