Essentially, any operator modifying Blender data should enable this
`UNDO` option, else bad things (corruption, crashes...) are likely to
happen.
* Added a new Operator example to explain this topic.
* Updated some existing Operator examples that were not correct anymore.
* Added a new small section in the gotchas page linking to it.
* Added also short reminder about this in the `UNDO` 'tooltip'
description itself.
Related to #77557.
69a7948575