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>