meson: docs: Install all manpages, not just ones in man1
In
f13eb16485f I made a mistake leading to only man1 being installed. I will
report a bug suggesting that meson warn about mistakes of this sort.
Reported-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/ZUU5pRQO6ZUeBsi6@msg.df7cb.de
Backpatch: 16-, where the meson build was introduced