Add information to Breaking Change docs about how default can cause permadeletes

[SarahFrench]

I recently reviewed a fix for a permadelete that occurred by adding a field that was ForceNew with a Default value: https://github.com/GoogleCloudPlatform/magic-modules/pull/9870

After seeing this page I figured it could be explicitly called out as an issue to watch out for; adding a new field with a default from the beginning may not feel like a breaking change, but it only is if it's immutable.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR hasn't generated any diffs, but I'll let you know if a future commit does.

[SarahFrench]

Closing my own PR as stale, when/if I have time I'll reopen!