I'm not 100% sure what this sentence is saying, but I'm pretty sure it needs either commas or parentheses to break it up a little. I think parens make the most sense in this case assuming I am reading it correctly. Here's the original sentence and proposed edit in plaintext:
ORIG
"...persistent objects which were removed from a collection or in some cases a scalar attribute may also be pulled into the Session of a parent object;..."
EDIT
"...persistent objects which were removed from a collection (or in some cases a scalar attribute) may also be pulled into the Session of a parent object;..."
``save-update`` has the possibly surprising behavior which is that
persistent objects which were *removed* from a collection
-or in some cases a scalar attribute
+(or in some cases a scalar attribute)
may also be pulled into the :class:`.Session` of a parent object; this is
so that the flush process may handle that related object appropriately.
This case can usually only arise if an object is removed from one :class:`.Session`