Update or insert: added text about using a query as the select condition (from newsgroup post)

took part of the text from this post
https://groups.google.com/forum/?fromgroups=#!searchin/web2py/update_or_insert$20multiple$20fields/web2py/cgywCCKZQjk/mS6cmOAXOO4J

which talks about (re update_or_insert)
"can be a query

condition = (db.tag_records.name==auth.user_id)&(db.tag_records.tag==tag)"