Doc: note clustering change (#472)

# Description

Add a warning about strict cluster host checking

## Type of change

Please delete options that are not relevant.

- [x] Documentation update (Doc only change)

## How Has This Been Tested?

> **_NOTE:_** All functional changes should accompany corresponding
tests (unit tests, functional tests etc).

Please describe the addition/modification of tests done to verify this
change. Please also list any relevant details for your test
configuration.

## Contributor's Checklist

Please check that you have:

- [ ] self-reviewed the code in this PR.
- [ ] added code comments, particularly in hard-to-understand areas.
- [ ] updated the user documentation with corresponding changes.
- [ ] added tests to verify effectiveness of this change.

Signed-off-by: Peter Sabaini <peter.sabaini@canonical.com>

docs/reference/release-notes.rst

@@ -24,6 +24,18 @@ Highlights
 -  Updated dependencies, based off of Ubuntu 24.04
 -  Various fixes and documentation improvements
 
+
+Important Changes
+-----------------
+
+For added security, MicroCeph now checks hostnames upon cluster
+joining. This means that the name used when running `microceph cluster
+add <name>` must match the hostname of the node where `microceph
+cluster join` is being run. If the hostname does not match joining the
+node will fail, and log a message `Joining server certificate SAN does
+not contain join token name` to syslog.
+
+
 Known Issues
 ------------