patch 8.1.2238: error in docs tags goes unnoticed

Problem:    Error in docs tags goes unnoticed.
Solution:   Adjust tags build command. (Ken Takata, closes #5158)
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index 4a1ea03..443a27a 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -323,7 +323,7 @@
 # Use Vim to generate the tags file.  Can only be used when Vim has been
 # compiled and installed.  Supports multiple languages.
 vimtags: $(DOCS)
-	$(VIMEXE) -u NONE -esX -c "helptags ++t ." -c quit
+	$(VIMEXE) -eX -u doctags.vim
 
 # Use "doctags" to generate the tags file.  Only works for English!
 tags: doctags $(DOCS)
diff --git a/runtime/doc/doctags.vim b/runtime/doc/doctags.vim
new file mode 100644
index 0000000..7dfbcc0
--- /dev/null
+++ b/runtime/doc/doctags.vim
@@ -0,0 +1,6 @@
+" This script makes a tags file for help text.
+"
+" Usage: vim -eX -u doctags.vim
+
+helptags ++t .
+qa!