docs: pdfdocs: Prevent column squeezing by tabulary

Setting a reasonable width to \tymin prevents column squeezing
by tabulary.
Width of 20em works well in almost all the tables still in the
ascii-art format.

Excerpt from tabulary package documentation at [1]:

    To stop very narrow columns being too 'squeezed' by this process
    any columns that are narrower than \tymin are set to their natural
    width.

[1]: https://mirrors.ctan.org/macros/latex/contrib/tabulary/tabulary.pdf

Note: Sphinx has its own default value of \tymin set in
sphinxlatextables.sty (Sphinx 4.0.2) and sphinxmulticell.sty
(Sphinx 2.4.4) as follows:

    \setlength{\tymin}{3\fontcharwd\font`0 }

, which is not sufficient for kernel-doc.

Tested against Sphinx versions 2.4.4 and 4.0.2.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/277d68fa-c96a-0ccb-6ce0-4d314851d9fe@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -353,6 +353,8 @@
 
     # Additional stuff for the LaTeX preamble.
     'preamble': '''
+	% Prevent column squeezing of tabulary.
+	\\setlength{\\tymin}{20em}
         % Use some font with UTF-8 support with XeLaTeX
         \\usepackage{fontspec}
         \\setsansfont{DejaVu Sans}