Document caveats around $allMatch, $elemMatch, etc.

As documented in [another issue](https://github.com/apache/couchdb/issues/2236), some selectors incur performance costs [because they do not map to how views work](https://github.com/apache/couchdb/issues/2236#issuecomment-538928959). However, only `$regex` is identified as not functioning with indices. All such operators that incur these unexpected costs should be documented.

## Expected Behavior

Performance caveats for Mango operators are documented clearly.

## Current Behavior

Performance caveats are only documented for `$regex`.

Start with the Mango selector documentation and the linked issue #2236, then compare the documented $regex behavior with operators such as $allMatch and $elemMatch. Document the relevant performance and index-use caveats clearly for all affected operators.

Written by the indexing model from the issue text.