feat(features): Document trial plan tiers (#1075)

* Document trial plan properties
* Clarify plan flags

src/docs/feature-flags/index.mdx

@@ -362,15 +362,23 @@ Enabling a feature for customers on specific plans can be done by creating a
 segment using plan properties. There are a few plan related properties you can
 use:
 
-* `subscription_is-free` Whether or not the account is on a free plan.
+* `subscription_is-free` Whether or not the main account subscription is on a
+  free plan.
 * `subscription_channel` Which billing channel an organization is on. The values
   of this can be found on the `Subscription` model.
-* `subscription_plan-family` This is one of `free`, `team` or `business` or
-  None.
-* `subscription_plan-tier` This is the plan tier eg. `am1`.
+* `subscription_plan-family` This is the plan family of the main subscription.
+  One of `free`, `team` or `business` or None.
+* `subscription_plan-tier` This is the plan tier of the main subscription eg. `am1`.
+* `subscription_plan-trial-plan-family` Family for the plan trial, if one exists.
+* `subscription_plan-trial-plan-tier` Tier for the plan trial, if one exists.
 * `subscription_plan` This property is deprecated and should not be used in new
   feature flags.
 
+_Note:_ Some customers may have both a regular plan for their main subscription
+_and_ a plan trial. If you need to target a plan tier, (e.g., `"am2"` you
+should create Flagr constraints for both the `subscription_plan-tier` property
+_and_ the `subscription_plan-trial-plan-tier` property.)
+
 ![enable plan subset](../img/flagrPlanSubset.png)
 
 ### Releasing to organizations incrementally