DOC: config: recommend single quoting passwords

DOC: config: recommend single quoting passwords

Suggests single quoting passwords and update examples to avoid unexpected
behaviors due to special characters.

Should be backported to stable versions.

Link: https://discourse.haproxy.org/t/enhance-documentation-for-insecure-passwords-and-invald-characters/11959
(cherry picked from commit 9432e7d6887ade0db69947f343c5b5535a33e303)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit 456d7dc170fd4a3e6541cc5816810f398efe2d07)
[cf: context adjustment]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>