doc: config: include existing git-hook(1) section

It is already included in git-hook(1) but missing from git-config(1).

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.adoc

@@ -451,6 +451,8 @@ include::config/guitool.adoc[]
 
 include::config/help.adoc[]
 
+include::config/hook.adoc[]
+
 include::config/http.adoc[]
 
 include::config/i18n.adoc[]