DOC: config: Specify %Ta is only available in HTTP mode

DOC: config: Specify %Ta is only available in HTTP mode

%Ta format can only be used in HTTP mode but it was not specify in the
configuration manual.

This patch should fix the issue #1317.

(cherry picked from commit 3010e00e1cc8388c2c01a43c597aa3ca07bff487)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit f601a5cbff140f7571107ff3d21cae419145ac43)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 3f1771667ced4000c3a00091231ea03d9427aea2)
Signed-off-by: Willy Tarreau <w@1wt.eu>