The 'documentation' option also guarded the man page build. Instead
if skipping the whole docs subdir skip the specific gtkdoc calls, so that the
man page build still works.
This brings it in line with the gtk3 meson build.
Unconditionally putting 'gdkwayland_inc' in src_dir argument of gtkdoc
call tells gtkdoc-scan to scan source files in a non-existent build
directory, gdk/wayland. To avoid causing build failure when a specific
backend is disabled, we should include directories conditionally.
We need to reference the types file directly, because it won't be copied
into the builddir by Meson — except for GTK, which needs to generate its
own types file using configure_file().
