runtime(doc): clarify behaviour of set maxcombine=0

related: #17400

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 0f8d063..47f56c0 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -5719,6 +5719,7 @@
 	Maximum value is 6.
 	Even when this option is set to 2 you can still edit text with more
 	combining characters, you just can't see them.  Use |g8| or |ga|.
+	When set to 0, you will not be able to see any combining characters.
 	See |mbyte-combining|.
 
 						*'maxfuncdepth'* *'mfd'*