Monitor parameter changes: Add note to save callback handle (backport #3102)

[mergify]

This is an automatic backport of pull request #3102 done by Mergify.
Cherry-pick of 1d24955 has failed:

On branch mergify/bp/foxy/pr-3102
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	deleted by us:   source/Tutorials/Intermediate/Monitoring-For-Parameter-Changes-CPP.rst

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[clalancette]

Closing this one, as it doesn't seem to be available for Foxy.