T-SQL in SQL Server does allow commenting.  In Access, you can write T-SQL in pass-through queries and write comments inside them.  Comments must precede with -- or enclosed with /* and */ as shown below.

If you are using the Monaco Editor, you can use comments before the initial SQL statement but not in the body.

Donald   I didn't know that.  I'm using the stand-alone version of Access, so I don't get Monaco editor.

It's still pretty new. I think there are still some issues they are working on but I  haven't really used them other than experimentation yet.

Amir, yes, but it depends on which SQL editor and database engine you're using. Donald is correct: with the Monaco editor in Access for Microsoft 365, you can put -- comments or /* block comments */ at the very beginning of a regular Access query, before SELECT. You cannot put them between clauses or after the SQL statement.

Kevin's screenshot shows a pass-through query running SQL Server's T-SQL, which allows comments within the query too. That's a different case from a regular Access query.

If you're using the classic Access SQL editor, keeping your notes in Notepad++ is a valid option. Thanks, Kevin and Donald, for covering both cases.