runtime(doc): clarify when text properties are cleared

related: #15030

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/textprop.txt b/runtime/doc/textprop.txt
index a02a3d5..6b46e06 100644
--- a/runtime/doc/textprop.txt
+++ b/runtime/doc/textprop.txt
@@ -530,7 +530,7 @@
 properties below the changed text, so that they still highlight the same text,
 thus you don't need to update these.
 
-
+							*text-prop-cleared*
 Text property columns are not updated or copied: ~
 
 - When setting the line with |setline()| or through an interface, such as Lua,