diff --git a/runtime/doc/popup.txt b/runtime/doc/popup.txt
index 6645a7b..dcde185 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -271,6 +271,11 @@
 		'buftype' set to "popup".  That buffer will be wiped out once
 		the popup closes.
 
+		if {what} is a buffer number and loading the buffer runs into
+		an existing swap file, it is silently opened read-only, as if
+		a |SwapExists| autocommand had set |v:swapchoice| to 'o'.
+		This is because we assume the buffer is only used for viewing.
+
 		{options} is a dictionary with many possible entries.
 		See |popup_create-arguments| for details.