There are two different things in Checkm called custom host/service notes.
There are the rulesets Notes URL for Hosts and Notes URL for Services. If you create those rules and provide links to a Wiki or documentation tool, in the burger menu of a host oder service view appears a link to the external website.
In earlier Checkmk versions, there was an embedded DokuWiki which could be used as link target. This was dropped, I believe in Checkmk 2.0. But you can easily link to an external DokuWiki oder other Wiki software.
And there is the Custom host notes and Custom service notes field which appear at the bottom of host and service views. The contents of those fields are taken from the ~/etc/check_mk/notes directory of a Checkmk site. However, I cannot find any documentation about this feature since the relaunch of the Checkmk web site. Maybe it is deprecated?
Moreover, it became less useful starting from Checkmk 2.1. In earlier versions, you could use wildcards in file names, e.g. a custom note in the file ~/etc/check_mk/notes/services/*/Memory was valid for the “Memory” check of any host. Since Checkmk 2.1, you can no longer use wildcards in directory names. I don’t know if this is a bug or intended behaviour.