DOC: better describes how to configure a fallback crt

DOC: better describes how to configure a fallback crt

A default certificate is always the first one declared in the bind line,
either from `crt` or from `crt-line` option. This commit updates the
description of how to configure a fallback certificate, clarifying that
it needs to be the first one of the bind line.

Should be merged as far as the first SNI filter implementation.

(cherry picked from commit aa8fcc4692b8c2afda455199a694067fea9e9262)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>