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

4b10302 ("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.

Author: wlallemand

doc/configuration.txt

@@ -1272,8 +1272,8 @@ Four other directives are provided to report some status:
   - .warning "message" : emit this message at level WARNING
   - .alert "message"   : emit this message at level ALERT
 
-Messages emitted at level WARNING may cause the process to fail to start if the
-"strict-mode" is enabled. Messages emitted at level ALERT will always cause a
+Messages emitted at level WARNING may cause the process to fail to start if
+"zero-warning" is enabled. Messages emitted at level ALERT will always cause a
 fatal error. These can be used to detect some inappropriate conditions and
 provide advice to the user.