patch 8.1.0553: it is not easy to edit a script that was sourced

Problem:    It is not easy to edit a script that was sourced.
Solution:   Add a count to ":scriptnames", so that ":script 40" edits the
            script with script ID 40.

runtime/doc/repeat.txt

@@ -334,6 +334,9 @@ For writing a Vim script, see chapter 41 of the user manual |usr_41.txt|.
 			{not in Vi} {not available when compiled without the
 			|+eval| feature}
 
+:scr[iptnames][!] {scriptId}			*:script*
+			Edit script {scriptId}.  Suggested name is ":script".
+
 						*:fini* *:finish* *E168*
 :fini[sh]		Stop sourcing a script.  Can only be used in a Vim
 			script file.  This is a quick way to skip the rest of