Enhance comments in task_queue.proto for clarity (#668)

Added comments to clarify the purpose of TaskQueueKind.

_**READ BEFORE MERGING:** All PRs require approval by both Server AND
SDK teams before merging! This is why the number of required approvals
is "2" and not "1"--two reviewers from the same team is NOT sufficient.
If your PR is not approved by someone in BOTH teams, it may be summarily
reverted._

<!-- Describe what has changed in this PR -->
**What changed?**
Added comments to `TaskQueueKind` clarifying that the field is for
wether a task queue is sticky and therefore only applies to workflow
tasks; thus will be unspecified if it is for any non-workflow task

<!-- Tell your future self why have you made these changes -->
**Why?**
Requested by Temporal Cloud Customer for Clarity

Author: billrich2001