Add alert to "avoid persisting detached entities"

[Pictor13]

The alert is hidden into the code

orm/lib/Doctrine/ORM/EntityManager.php

Lines 619 to 620 in 8c259ea

	* NOTE: The persist operation always considers entities that are not yet known to
	* this EntityManager as NEW. Do not pass detached entities to the persist operation.

while it's useful to be mentioned in the documentation, the same way it is done for the documentation from Doctrine ODM, as caution alert at the end of the section.

[greg0ire]

Please kindly squash your commits together. If you don't, we'll try to remember to do it for you but it's best if you save us this trouble.

How to do that?

1. git rebase -i origin/2.7, assuming origin is a git remote that points to this repository, and not your fork. If you're not sure what your remotes are, run git remote -vvv, there should be your fork and the holy/reference/base/origin/whatever-you-call-it repository.
2. A window will show up with many lines, replace pick with fixup on every line but the first one
3. Close your editor, git should do its magic, and you should end up with one commit
4. Use git push --force to overwrite what you already push. Don't forget the --force option otherwise git will try to merge both things together.

[Pictor13]

Thank you for the instructions @greg0ire!
(even though I usually don't like to force push on PRs 😅)