DOC: config: prefer-last-server: add notes for non-deterministic algorithms

DOC: config: prefer-last-server: add notes for non-deterministic algorithms

Add some notes which load-balancing algorithm can be considered as
deterministic or non-deterministic and add some examples for each type.
This was asked via mailing list to clarify the usage of
prefer-last-server option.

This can be backported to all stable versions.

(cherry picked from commit cdb2f8d780a27778cc23669693a9910e07135e8a)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit d6ca14b764039ed1c5d21d2fa83706996fd0a717)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>