git-sh-i18n--envsubst: add SYNOPSIS section to the documentation

Change the documentation for the git-sh-i18n--envsubst program to
include a SYNOPSIS section. Include the invocation of the program from
git-sh-i18n.sh.

Not having a SYNOPSIS section caused the "doc" target to fail on
Centos 5.5 with asciidoc 8.2.5, while building with 8.6.4 on Debian
works just fine.

The relevant error was:

    ERROR: git-sh-i18n--envsubst.txt: line 9: second section must be named SYNOPSIS
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-sh-i18n--envsubst.txt

@@ -5,6 +5,16 @@ NAME
 ----
 git-sh-i18n--envsubst - Git's own envsubst(1) for i18n fallbacks
 
+SYNOPSIS
+--------
+[verse]
+eval_gettext () {
+	printf "%s" "$1" | (
+		export PATH $('git sh-i18n--envsubst' --variables "$1");
+		'git sh-i18n--envsubst' "$1"
+	)
+}
+
 DESCRIPTION
 -----------