DOC: configuration: add details on prefer-client-ciphers

DOC: configuration: add details on prefer-client-ciphers

prefer-client-ciphers does not work exactly the same way when used with
a dual algorithm stack (ECDSA + RSA). This patch details its behavior.

This patch must be backported in every maintained version.

Problem was discovered in #2988.

(cherry picked from commit 370a8cea4a2680cf27d5be61163bada27d541347)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 5000d32a2488a47cf817bebcf023312510d0cddc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>