"New in version 1.6."

This is a huge pythonism. 5(?)years later, this is still touted as a brand new feature.

• The version number should be link to the release notes
• Was it 1.6 or 2.6 that was just released? Or 3.6? Can I safely use this feature?
  • Takes up way to much precious screen real-estate for no gain
  • We don't maintain multiple major branches so why do I care it was added before current stable major release?
• Why are the documents both versioned and changes kept inline? pick one.

How inefficient is nested $or (which is the second everybody-must-know about this feature - and you need to know it was added in 2.0).

This modified query will not use the index on price nor the index on sale.

Why not? How do I make it? Does it use any index?

The example "document" is also weird.
The inventory contains a mix of items and already sold items?
Is it really good practice to create an index on a boolean field?
Is 'quantity' to long fieldname so we have to use 'qty'?

If $or includes a $text query, all clauses in the $or array must be indexed.

Does it have to be the same index? Can the index be of different type? Why is $text so special?