[TEP-0135] Improve workspace related documentation

This commit updates the `Workspace` related documentation after introducing the `coschedule` feature flag.
This commit also elaborates more on `PersistentVolume` Availability Zone scheduling conflict and calls out restrictions
of using multiple PVC based workspaces in a `PipelineTaskRun` in different coschedule modes.

/kind documentation

docs/workspaces.md

@@ -366,6 +366,9 @@ to define when a `Task` should be executed. For more information, see the [`runA
 When a `PersistentVolumeClaim` is used as volume source for a `Workspace` in a `PipelineRun`,
 an Affinity Assistant will be created. For more information, see the [`Affinity Assistants` documentation](affinityassistants.md).
 
+**Note**: When `coschedule` is set to `workspaces` or `disabled`, it is not allowed to bind multiple [`PersistentVolumeClaim` based workspaces](#using-persistentvolumeclaims-as-volumesource) to the same `TaskRun` in a `PipelineRun` due to potential Availability Zone conflicts.
+See more details in [Availability Zones](#availability-zones).
+
 #### Specifying `Workspaces` in `PipelineRuns`
 
 For a `PipelineRun` to execute a `Pipeline` that includes one or more `Workspaces`, it needs to
@@ -601,6 +604,13 @@ the storage solution that you are using.
 * `ReadWriteMany` is the least commonly available Access Mode. If you use this access mode and these volumes are available
   to all Nodes within your cluster, you may want to disable the Affinity Assistant.
 
+### Availability Zones
+`Persistent Volumes` are "zonal" in some cloud providers like GKE (i.e. they live within a single Availability Zone and cannot be accessed from a `pod` living in another Availability Zone). When using a workspace backed by a `PersistentVolumeClaim` (typically only available within a Data Center), the `TaskRun` `pods` can be scheduled to any Availability Zone in a regional cluster. This results in potential Availability Zone scheduling conflict when two `pods` requiring the same Volume are scheduled to different Availability Zones.
+
+To avoid such conflict in `PipelineRuns`, Tekton provides [Affinity Assistants](affinityassistants.md) which schedule all `TaskRun` `pods` or all `TaskRun` sharing a `PersistentVolumeClaim` in a `PipelineRUn` to the same Node depending on the `coschedule` mode.
+
+Specifically, for users use zonal clusters like GKE or use `PersistentVolumeClaim` in ReadWriteOnce access modes, please set `coschedule: workspaces` to schedule each of the `TaskRun` `pod` to the same zone as the associated `PersistentVolumeClaim`. In addition, for users want to bind multiple `PersistentVolumeClaims` to a single `TaskRun`, please set `coschedule: pipelineruns` to schedule all `TaskRun` `pods` and `PersistentVolumeClaim` in a `PipelineRun` to the same zone.
+
 ## More examples
 
 See the following in-depth examples of configuring `Workspaces`: