patch 9.0.1231: completion of :runtime does not handle {where} argument

Problem:    Completion of :runtime does not handle {where} argument.
Solution:   Parse the {where} argument. (closes #11863)
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index a888ae0..4fa6b34 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -3528,6 +3528,7 @@
 		messages	|:messages| suboptions
 		option		options
 		packadd		optional package |pack-add| names
+		runtime		runtime file names |:runtime|
 		scriptnames	sourced script names |:scriptnames|
 		shellcmd	Shell command
 		sign		|:sign| suboptions