patch 8.2.3162: Vim9: argument types are not checked at compile time

Problem:    Vim9: argument types are not checked at compile time.
Solution:   Add more type checks. (Yegappan Lakshmanan, closes #8560)
diff --git a/runtime/doc/channel.txt b/runtime/doc/channel.txt
index 6e5cfc8..4049f22 100644
--- a/runtime/doc/channel.txt
+++ b/runtime/doc/channel.txt
@@ -852,7 +852,7 @@
 job_getchannel({job})					 *job_getchannel()*
 		Get the channel handle that {job} is using.
 		To check if the job has no channel: >
-			if string(job_getchannel()) == 'channel fail'
+			if string(job_getchannel(job)) == 'channel fail'
 <
 		Can also be used as a |method|: >
 			GetJob()->job_getchannel()