patch 9.0.0285: it is not easy to change the command line from a plugin

Problem:    It is not easy to change the command line from a plugin.
Solution:   Add setcmdline(). (Shougo Matsushita, closes #10869)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 57cdb11..e2d32f9 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1038,6 +1038,7 @@
 	getcmdpos()		get position of the cursor in the command line
 	getcmdscreenpos()	get screen position of the cursor in the
 				command line
+	setcmdline()		set the current command line
 	setcmdpos()		set position of the cursor in the command line
 	getcmdtype()		return the current command-line type
 	getcmdwintype()		return the current command-line window type