gh-76607: Improve documentation of the tkinter cursor option (GH-152481)

Link to the cursors(3tk) manual page for the available cursor names and
note that they are platform-dependent.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/tkinter.rst

@@ -737,6 +737,8 @@ Here are some examples of typical usage::
    myapp.mainloop()
 
 
+.. _Tk-option-data-types:
+
 Tk option data types
 ^^^^^^^^^^^^^^^^^^^^
 
@@ -770,12 +772,16 @@ color
    represent any legal hex digit.  See page 160 of Ousterhout's book for details.
 
 cursor
-   The standard X cursor names from :file:`cursorfont.h` can be used, without the
-   ``XC_`` prefix.  For example to get a hand cursor (``XC_hand2``), use the
-   string ``"hand2"``.  You can also specify a bitmap and mask file of your own.
+   The name of the mouse cursor to display while the pointer is over the widget.
+   Tk provides a portable set of cursor names available on all platforms
+   (for example ``"arrow"``, ``"watch"``, ``"cross"``, or ``"hand2"``);
+   the standard X cursor names from :file:`cursorfont.h` may also be used,
+   without the ``XC_`` prefix (so ``XC_hand2`` becomes ``"hand2"``).
+   The full list of names, including the platform-specific ones,
+   is given in the :manpage:`cursors(3tk)` manual page.
+   You can also specify a bitmap and mask file of your own.
    On Windows a cursor file (:file:`.cur` or :file:`.ani`) may be used directly,
    giving its path preceded with an ``@``, as in ``"@C:/cursors/bart.ani"``.
-   See page 179 of Ousterhout's book.
 
 distance
    Screen distances can be specified in either pixels or absolute distances.

Doc/library/tkinter.ttk.rst

@@ -116,9 +116,10 @@ All the :mod:`!ttk` Widgets accept the following options:
 |           | read-only, and may only be specified when the window is      |
 |           | created.                                                     |
 +-----------+--------------------------------------------------------------+
-| cursor    | Specifies the mouse cursor to be used for the widget. If set |
-|           | to the empty string (the default), the cursor is inherited   |
-|           | from the parent widget.                                      |
+| cursor    | Specifies the mouse cursor to be used for the widget.  See   |
+|           | the *cursor* option type under :ref:`Tk-option-data-types`.  |
+|           | If set to the empty string (the default), the cursor is      |
+|           | inherited from the parent widget.                            |
 +-----------+--------------------------------------------------------------+
 | takefocus | Determines whether the window accepts the focus during       |
 |           | keyboard traversal. 0, 1 or an empty string is returned.     |