Documentation doesn't build anymore on old asciidoc due to i18n series

I can submit a patch for this later, just thought I'd note it and solicit feedback on the best solution to this.

On my Debian box with asciidoc 8.6.4 Git builds just fine from master.

But on a Centos 5.5 box with asciidoc 8.2.5 I get:

    rm -f git-sh-i18n--envsubst.html+ git-sh-i18n--envsubst.html && \
            asciidoc -b xhtml11 -d manpage -f asciidoc.conf \
                     -a asciidoc7compatible -a no-inline-literal -a
git-asciidoc-no-roff -agit_version=1.7.5.3.367.ga9930 -o
git-sh-i18n--envsubst.html+ git-sh-i18n--envsubst.txt && \
            mv git-sh-i18n--envsubst.html+ git-sh-i18n--envsubst.html
    ERROR: git-sh-i18n--envsubst.txt: line 9: second section must be
named SYNOPSIS
    make[1]: *** [git-sh-i18n--envsubst.html] Error 1

The solution is obvious: just add a SYNOPSIS section for consistency. I'll do that later, unless we want to provide some option (which old asciidoc may or may not support) to not have a SYNOPSIS there, which we arguably can just omit for purely internal tooling like this.

I'll submit patches soon for adding a SYNOPSIS unless we want to go that route.