patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)
diff --git a/runtime/doc/if_ruby.txt b/runtime/doc/if_ruby.txt
index 4523660..dc8349f 100644
--- a/runtime/doc/if_ruby.txt
+++ b/runtime/doc/if_ruby.txt
@@ -28,15 +28,15 @@
 :rub[y] {cmd}		Execute Ruby command {cmd}.  A command to try it out: >
 				:ruby print "Hello"
 
-:rub[y] << [endmarker]
+:rub[y] << [trim] [{endmarker}]
 {script}
 {endmarker}
 			Execute Ruby script {script}.
-			The {endmarker} after {script} must NOT be preceded by
-			any white space.
 
 			If [endmarker] is omitted, it defaults to a dot '.'
-			like for the |:append| and |:insert| commands.
+			like for the |:append| and |:insert| commands.  Refer
+			to |:let-heredoc| for more information.
+
 
 			This form of the |:ruby| command is mainly useful for
 			including ruby code in vim scripts.