gh-157140: Fix inaccuracies in the curses documentation (GH-157141)

Correct the descriptions of halfdelay(), termname(), window.scroll(),
window.insnstr() and window.insch().

Author: fedonman

Doc/library/curses.rst

@@ -274,9 +274,11 @@ Input options
 
    Used for half-delay mode, which is similar to cbreak mode in that characters
    typed by the user are immediately available to the program. However, after
-   blocking for *tenths* tenths of seconds, raise an exception if nothing has
-   been typed.  The value of *tenths* must be a number between ``1`` and ``255``.  Use
-   :func:`nocbreak` to leave half-delay mode.
+   blocking for *tenths* tenths of seconds, if nothing has been typed,
+   :meth:`~window.getch` returns ``-1``, while :meth:`~window.getkey` and
+   :meth:`~window.get_wch` raise :exc:`curses.error`.
+   The value of *tenths* must be a number between ``1`` and ``255``.
+   Use :func:`nocbreak` to leave half-delay mode.
 
 .. function:: meta(flag)
 
@@ -893,8 +895,10 @@ Querying the terminal
 
 .. function:: termname()
 
-   Return the value of the environment variable :envvar:`TERM`, as a bytes object,
-   truncated to 14 characters.
+   Return the name of the terminal in use, as a bytes object.
+   This is the name passed to :func:`newterm` or
+   :func:`setupterm`, or the value of the :envvar:`TERM` environment variable
+   if none was given.  Some curses libraries truncate it to 14 characters.
 
 .. function:: longname()
 
@@ -1227,7 +1231,8 @@ Adding and inserting text
    Insert character *ch* with attributes *attr* before the character under the
    cursor, or at ``(y, x)`` if specified.  All characters to the right of the
    cursor are shifted one position right, with the rightmost character on the
-   line being lost.  The cursor position does not change.
+   line being lost.
+   The cursor position does not change (after moving to *y*, *x*, if specified).
 
    .. versionchanged:: 3.16
       Wide and combining characters, and :class:`complexchar` cells, are now
@@ -1251,8 +1256,8 @@ Adding and inserting text
             window.insnstr(y, x, str, n[, attr])
 
    Insert a character string (as many characters as will fit on the line) before
-   the character under the cursor, up to *n* characters.   If *n* is zero or
-   negative, the entire string is inserted. All characters to the right of the
+   the character under the cursor, up to *n* characters.   If *n* is negative,
+   the entire string is inserted. All characters to the right of the
    cursor are shifted right, with the rightmost characters on the line being lost.
    The cursor position does not change (after moving to *y*, *x*, if specified).
 
@@ -1871,8 +1876,9 @@ Output options
 .. method:: window.scroll([lines=1])
 
    Scroll the screen or scrolling region.  Scroll upward by *lines* lines if
-   *lines* is positive, or downward if it is negative.  Scrolling has no effect
-   unless it has been enabled for the window with :meth:`scrollok`.
+   *lines* is positive, or downward if it is negative.
+   Raise :exc:`curses.error` unless scrolling has been enabled for the window
+   with :meth:`scrollok`.
 
 .. method:: window.setscrreg(top, bottom)