Document let support for aggregation

PYTHON-2741 added test but we forgot to update the aggregate docs to mention this option. We should add this to Collection.aggregate and Database.aggregate:
```
+ - `let` (dict): A dict of parameter names and values. Values must be
+ constant or closed expressions that do not reference document
+ fields. Parameters can then be accessed as variables in an
+ aggregate expression context (e.g. ``"$$var"``). This option is
+ only supported on MongoDB >= 5.0.
```