DOC: configuration: confuse "strict-mode" with "zero-warning"

DOC: configuration: confuse "strict-mode" with "zero-warning"

4b10302fd8 ("MINOR: cfgparse: implement a simple if/elif/else/endif
macro block handler") introduces a confusion between "strict-mode" and
"zero-warning".

This patch fixes the issue by changing "strict-mode" by "zero-warning"
in section 2.4. Conditional blocks.

Must be backported as far as 2.4.

(cherry picked from commit e0ec01849fc6eff6a6e9ac67e23dea03d779fe70)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 98d4d720e11ef808e849bba76e902fee7f6419d1)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 46cf247231358866b5c6c03e0176f7e5290a2492)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>