patch 8.1.1714: cannot preview a file in a popup window

Problem:    Cannot preview a file in a popup window.
Solution:   Add the 'previewpopup' option.
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index ad50bfc..7549ed8 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -5627,7 +5627,16 @@
 			{not available when compiled without the |+windows| or
 			|+quickfix| features}
 	Default height for a preview window.  Used for |:ptag| and associated
-	commands.  Used for |CTRL-W_}| when no count is given.
+	commands.  Used for |CTRL-W_}| when no count is given.  Not used when
+	'previewpopup' is set.
+
+					*'previewpopup'* *'pvp'*
+'previewpopup' 'pvp'	string (default empty)
+			global
+			{not available when compiled without the |+windows|,
+			|+textprop| or |+quickfix| feature}
+	When not empty a popup window is used for commands that would open a
+	preview window.  See |preview-popup|.
 
 					*'previewwindow'* *'nopreviewwindow'*
 					*'pvw'* *'nopvw'* *E590*
diff --git a/runtime/doc/windows.txt b/runtime/doc/windows.txt
index 2c0cdc3..1c6702f 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -864,7 +864,15 @@
 set in the preview window to be able to recognize it.  The 'winfixheight'
 option is set to have it keep the same height when opening/closing other
 windows.
-
+						*preview-popup*
+Alternatively, a popup window can be used by setting the 'previewpopup'
+option.  When set, it overrules the 'previewwindow' and 'previewheight'
+settings.  The option is a comma separated list of values:
+	height		maximum height of the popup
+	width		maximu width of the popup
+Example: >
+	:set previewpopup=height:10,width:60
+<
 						*:pta* *:ptag*
 :pta[g][!] [tagname]
 		Does ":tag[!] [tagname]" and shows the found tag in a