patch 8.1.1329: plans for popup window support are spread out

Problem:    Plans for popup window support are spread out.
Solution:   Add a first version of the popup window help.
diff --git a/runtime/doc/help.txt b/runtime/doc/help.txt
index c8ff75a..c2c9c1c 100644
--- a/runtime/doc/help.txt
+++ b/runtime/doc/help.txt
@@ -143,6 +143,7 @@
 |remote.txt|	using Vim as a server or client
 |term.txt|	using different terminals and mice
 |terminal.txt|	Terminal window support
+|popup.txt|	popop window support
 
 Programming language support ~
 |indent.txt|	automatic indenting for C and other languages