DOC: explain the relation between pool-low-conn and tune.idle-pool.shared

DOC: explain the relation between pool-low-conn and tune.idle-pool.shared

Disabling idle-pool sharing can result in awful performance in presence
of a not so high number of threads, because the number of available idle
connections will be shared among threads, resulting in most of them
abandonning their connections after a request is done if there are already
enough total available. This is a case where pool-low-conn ought to be
used to preserve a number of connections for each thread, but this relation
isn't obvious as is. Let's add mentions about this with both keywords.

(cherry picked from commit 0784db8566b5c4eea5b18bf533ef978a1909b97a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>