patch 8.2.5088: value of cmod_verbose is a bit complicated to use

Problem:    Value of cmod_verbose is a bit complicated to use.
Solution:   Use zero for not set, value + 1 when set. (closes #10564)
diff --git a/src/structs.h b/src/structs.h
index 7fe97fd..9e40928 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -662,8 +662,8 @@
     regmatch_T	cmod_filter_regmatch;	// set by :filter /pat/
     int		cmod_filter_force;	// set for :filter!
 
-    int		cmod_verbose;		// non-zero to set 'verbose', -1 is
-					// used for zero override
+    int		cmod_verbose;		// 0 if not set, > 0 to set 'verbose'
+					// to cmod_verbose - 1
 
     // values for undo_cmdmod()
     char_u	*cmod_save_ei;		// saved value of 'eventignore'