Document how to use where operator with parent context

[Sylvan Le Deunff]

Documentation gives example on how to use "where" operator with constants filters (like "1" and "John", in example below)

$.customers.where($.orders.len() >= 1 or name = "John")

In real life applications, users will most likely need to use variables defined from context.
(See: https://bugs.launchpad.net/yaql/+bug/1790115)
I couldn't find how to do this in the documentation, only by reading the code.

I suggest adding the following example in Quickstart (https://opendev.org/openstack/yaql/src/branch/master/README.rst)

let(target_age => $.target_age)->$.users.where($.age=$target_age)

And maybe somewhere else adding a brief description of operation "context pass" (->) which is barely mentioned in the doc and not explained at all.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/yaql/+/886338