Add more descritpion, what get_ticket_id.exs does

scripts/get_ticket_id.exs

@@ -1,5 +1,14 @@
 # This script is used by .github/actions/close_issue/action.yml and it shouldn't be used in any other places.
 
+# It expects:
+#   - output from `$ gh project item-list <project_id> --owner <owner> --format json --limit <limit>` command
+#     on standard input
+#   - URL of issue, that corresponds to one of the tickets included in the JSON data returned from the command
+#     above. This URL should be passed as an argument in argv
+# And prints `TICKET_ID <id>` on standard output, where `<id>` is id of project item corresponding to issue
+# with the specified URL. Note, that beyond this, stdout can also contain some logs from `Mix.install/1`, e.g.
+# `Resolving Hex dependencies...`.
+
 Mix.install(json: "~> 1.4.1")
 
 [issue_url] = System.argv()