patch 8.0.1649: no completion for argument list commands

Problem:    No completion for argument list commands.
Solution:   Add arglist completion. (Yegappan Lakshmanan, closes #2706)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 387186f..e5f0b10 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -4469,6 +4469,7 @@
 		specifies what for.  The following completion types are
 		supported:
 
+		arglist		file names in argument list
 		augroup		autocmd groups
 		buffer		buffer names
 		behave		:behave suboptions