save_condition's User Guide documentation perhaps misleading?

Hi guys,

As usual, I hope I am not missing something so apologies in advance if I did.

To keep the story short the User Guide http://docs.mongoengine.org/guide/querying.html#atomic-updates says that operators such 'add_to_set' can be used with save() as long as save_condition is present.

After a couple of runs I realised that I can't used save_condition with modifiers but only with operators. For example when trying to use

mytest.save(save_condition={'add_to_set_myfield':[1,1,2]}

I was getting all the time:

mongoengine.errors.InvalidQueryError: Cannot resolve field "add_to_set"

Then I checked the save() method and saw that 'save_condition' is transformed into a query as shown on https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/document.py#L482

I then tracked down transform.query at https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/queryset/transform.py#L46 where I realised that only operators are allowed.

I was trying to update a record if and only if an underlying listfield is unique at saving time hence that when I read the doc I thought it could be done this way.

By the way on another matter, on the API Reference documentation http://docs.mongoengine.org/apireference.html#mongoengine.Document.save it would be great to add a line saying something like:

'force_insert' and 'save_condition' are incompatible as the former only applies to inserts while the latter on updates

This seems obvious for those who has been playing for a while with MongoEngine and MongoDB but for those who are new, it saves plenty of deduction time... And perhaps just raising an Exception at the beginning of the save() method

Start with the User Guide's atomic-updates section and the Document.save API reference, then compare their wording with document.py around line 482 and queryset/transform.py around line 46. Done means the documentation distinguishes save_condition operators from modifiers and explains its incompatibility with force_insert.