patch 9.0.1238: :runtime completion can be further improved

Problem:    :runtime completion can be further improved.
Solution:   Also complete the {where} argument values and adjust the
            completion for that. (closes #11874)
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 4fa6b34..265111d 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -3528,7 +3528,7 @@
 		messages	|:messages| suboptions
 		option		options
 		packadd		optional package |pack-add| names
-		runtime		runtime file names |:runtime|
+		runtime		|:runtime| completion
 		scriptnames	sourced script names |:scriptnames|
 		shellcmd	Shell command
 		sign		|:sign| suboptions