patch 8.1.1523: cannot show range of buffer lines in popup window

Problem:    Cannot show range of buffer lines in popup window.
Solution:   Add the "firstline" property. (closes #4523)
diff --git a/runtime/doc/popup.txt b/runtime/doc/popup.txt
index b80b875..7e56d35 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -87,7 +87,6 @@
 
 IMPLEMENTATION:
 - Why does 'nrformats' leak from the popup window buffer???
-- Option to set first line to display (useful for a preview window)
 - Disable commands, feedkeys(), CTRL-W, etc. in a popup window.
   Use NOT_IN_POPUP_WINDOW for more commands.
 - Add 'balloonpopup': instead of showing text, let the callback open a popup
@@ -396,6 +395,9 @@
 			padding.
 	minwidth	Minimum width of the contents, excluding border and
 			padding.
+	firstline	First buffer line to display.  When larger than one it
+			looks like the text scrolled up.  When out of range
+			the last buffer line will at the top of the window.
 	hidden		When TRUE the popup exists but is not displayed; use
 			`popup_show()` to unhide it.
 			{not implemented yet}