Pull/unset links to document upon deletion

[hadware]

Hello,

First of all, thanks for Beanie. This lib is absolutely fantastic and made me love MongoDB again :).

I'm an old-time user of mongoengine, and it had that one neat feature for ReferenceFields : the PULL/NULLIFY "reverse delete rule": https://docs.mongoengine.org/apireference.html#mongoengine.fields.ReferenceField

This would help you make sure that no links to other documents were left "dangling" when deleting a document, and prevented you the hassle of having to do post/pre-delete cleanup of references to a document.

From what I gathered, everything is already set up in Beanie for this feature to work. I could try and come up with a PR if you think you'd merge this.

I don't have a very definite idea of how the API would work, but the simplest way would be to add another DeleteRules-style parameter such as:

class LinksDeleteRules(Enum):
    DO_NOTHING = 0 # same behavior as now
    PULL_AND_UNSET = 1 # looks for all references for that document and either pulls them/unsets them

Side note: the documentation is a bit unclear on how the current DeleteRules work. At first i thought this was the intented behavior, and i'm not the only one. IMHO this would benefit a bit more elaboration. I can also make a small PR on the doc if you tell me how you'd like it explained.