gendocs.sh: Set default TOP_NODE_UP_URL in HTML output. Suggested by Gavin Smith <gavinsmith0123@gmail.com>. Reported by myglc2 <myglc2@gmail.com> in <http://bugs.gnu.org/22651>. * build-aux/gendocs.sh (MANUAL_TITLE, PACKAGE, EMAIL) (commonarg, dirargs, dirs, infoarg, generate_ascii) (generate_html, generate_info, generate_tex, outdir) (source_extra, split, srcfile, texarg): Move above 'version'. (htmlarg): Likewise, and add "-c TOP_NODE_UP_URL=/manual".