rectify docs around Tkinter

Will fix #36710

src/doc/en/faq/faq-usage.rst

@@ -93,8 +93,10 @@ How to get Sage's Python to recognize my system's Tcl/Tk install?
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 
 It may be that you have Tcl/Tk installed and that your system's Python
-recognizes it but Sage's Python does not. To fix that, install the
-tcl/tk development library. On Ubuntu, this is the command
+recognizes it but Sage's Python does not. Normally speaking, there is little
+need to build Sage's Python nowadays (anno 2023), but if you do, here it is.
+Make sure you installed the tcl/tk development library.  On Ubuntu, this is the
+command
 
 .. CODE-BLOCK:: shell-session
 
@@ -104,7 +106,7 @@ or something along that line. Next, reinstall Sage's Python:
 
 .. CODE-BLOCK:: shell-session
 
-    $ sage -f python3
+    $ make python3-clean python3-uninstall && make python3
 
 This will pick up the tcl/tk library automatically. After successfully
 reinstalling Sage's Python, from within the Sage command line interface,

src/doc/en/installation/source.rst

@@ -446,20 +446,34 @@ If you don't want conda to be used by sage, deactivate conda (for the current sh
   operating system, or its own compilers.
 
 
-Tcl/Tk
-^^^^^^
+Tcl/Tk (and system's Python)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you want to use `Tcl/Tk <https://www.tcl.tk/>`_ libraries in Sage, and you
+are going to use your OS's Python3 as Sage's Python, you merely need to install
+its **Tkinter** module.  On Lunix systems, it is usually provided by the
+**python3-tk** or a similarly named (e.g. **python3-tkinter**) package,
+which can be installed using::
+
+    $ sudo apt-get install python3-tk
 
-If you want to use `Tcl/Tk <https://www.tcl.tk/>`_ libraries in Sage, you need
-to install the Tcl/Tk and its development headers before building Sage.  Sage's
-Python will then automatically recognize your system's install of Tcl/Tk.
+or similar commands.
+
+Tcl/Tk (and Sage's own Python)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+If you want to use `Tcl/Tk <https://www.tcl.tk/>`_ libraries in Sage,
+and you are going to build Sage's Python from source, you need to install
+these, and the corresponding headers.
 On Linux systems, these are usually provided by the **tk** and **tk-dev**
 (or **tk-devel**) packages which can be installed using::
 
     $ sudo apt-get install tk tk-dev
 
 or similar commands.
 
+
+Sage's Python will then automatically recognize your system's install of Tcl/Tk.
 If you installed Sage first, all is not lost. You just need to rebuild
 Sage's Python and any part of Sage relying on it::