V3 docs: Upgrade Guide: filtering with inequality

- Technically, this also includes filtering with equality, specifically
  multiple filtering.

- We call out the differences between V3 and V2 inequality queries (no
  preamble, different operators)

[finishes #173277412]

Co-authored-by: Brian Cunnie <bcunnie@pivotal.io>
Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>

Author: Brian Cunnie

docs/v3/source/includes/upgrade_guide/conceptual_changes/_filtering.md

@@ -29,4 +29,25 @@ A few common filters have been also renamed in V3:
 
 <sup>1</sup> In V3, order is ascending by default. Prefix the `order_by` value with `-` to make it descending. For example, `?order_by=-name` would order a list of resources by `name` in descending order.
 
-Read more about [filtering in V3](#filtering).
+Filtering inequalities has changed in V3: V3 dispenses with the `q=` preamble,
+uses `created_ats` instead of `timestamp` and uses bracket operators (`[lt]`,
+`[gt]`, `[lte]`, `[gte]`). For example, to request all audit events occurring on
+New Year's Day, one would use the following query: `GET
+/v3/audit_events?created_ats[lt]=2020-01-02T00:00:00Z&created_ats[gt]=2019-12-31T23:59:59Z`.
+
+The corresponding V2 query would be `GET
+/v2/events?q=timestamp<2020-01-02T00:00:00Z&q=timestamp>2019-12-31T23:59:59Z`.
+
+Filtering on equality has also changed: V3 dispenses with the `q=` preamble and
+uses the pluralized field (e.g. `names`) on the left side of the equals sign.
+For filtering on inclusion in a set, V3 allows passing multiple values separated
+by commas.
+
+For example, to request the organizations by
+their name ("finance" and "marketing"), one would use the following query:
+`/v3/organizations?names=finance,marketing`
+
+The corresponding V2 query would be `GET
+/v2/organizations?q=name%20IN%20finance,marketing`
+
+Read more about [filtering in V3](#filters).