Add note about how to disable the subscription too?

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/sql-dropsubscription.html
Description:

This page has a note about how to set the subscription to not use a slot,
but it doesn't mention that the subscription needs to be disabled first. In
my case, I had to Google around and then do:

ALTER SUBSCRIPTION xxx disable;

Then I could set the slot_name. Then I could drop it.