Fix #10984 [DOC] document latex_additional_files behavior with .tex (#10993)

doc/usage/configuration.rst

@@ -2372,6 +2372,15 @@ These options influence LaTeX output.
    You have to make sure yourself that the filenames don't collide with those
    of any automatically copied files.
 
+   .. attention::
+
+      Filenames with extension ``.tex`` will automatically be handed over to
+      the PDF build process triggered by :option:`sphinx-build -M`
+      ``latexpdf`` or by :program:`make latexpdf`.  If the file was added only
+      to be ``\input{}`` from a modified preamble, you must add a further
+      suffix such as ``.txt`` to the filename and adjust accordingly the
+      ``\input{}`` command added to the LaTeX document preamble.
+
    .. versionadded:: 0.6
 
    .. versionchanged:: 1.2