DOC: management: certificate files must be sanitized before injection

DOC: management: certificate files must be sanitized before injection

A lot of people encounter problems when trying to inject a certificate
file which contains extra informations or empty lines.

This patch adds a paragraph and a sanitizing example.

Must be backported as far as 2.1.

(cherry picked from commit ed8bfadd8d17fc59b8a1f57bb2476cd7df1ce190)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 827347512a198e07f5acf9cb6767a06ef4898036)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>