DOC: config: recommend disabling libc-based resolution with resolvers

DOC: config: recommend disabling libc-based resolution with resolvers

Using both libc and haproxy resolvers can lead to hard to diagnose issues
when their bevahiour diverges; recommend using only one type of resolver.

Should be backported to stable versions.

Link: https://www.mail-archive.com/haproxy@formilux.org/msg45663.html
Co-authored-by: Lukas Tribus <lukas@ltri.eu>
(cherry picked from commit 4e20fab7ac85c0018e77a52657f773982f8c1875)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit e106c8f8f1068fba0923dd3348c6e7cb0bc499ab)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>