.github: improve formatting of the telemetry proposal template

Tweak the new telemetry proposal template added in CL 583496:
 - Shorten the description, as it is formatted on one conspicuously long
   line in the template picker.
 - Use folded style for label descriptions, as their line breaks cause
   the resulting paragraph to flow awkwardly.

Change-Id: I3089ac0717646e153765548d4bebd8d4751933b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/583916
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

.github/ISSUE_TEMPLATE/12-telemetry.yml

@@ -1,7 +1,5 @@
 name: Go Telemetry Proposals
-description: |
-  Modify the telemetry upload configuration, by adding new counters to upload,
-  or refining or removing existing upload configuration.
+description: Changes to the telemetry upload configuration
 title: "x/telemetry/config: proposal title"
 labels: ["Telemetry-Proposal"]
 projects: ["golang/29"]
@@ -9,11 +7,10 @@ body:
 - type: textarea
   attributes:
     label: Summary
-    description: |
+    description: >
       What change are you proposing to the upload configuration, and why?
       For new upload configuration, which new counters will be collected, what
       do they measure, and why is it important to collect them?
-      All counters must already be instrumented in the target program.
       Note that uploaded data must not carry sensitive user information.
       See [go.dev/doc/telemetry#proposals](https://go.dev/doc/telemetry#proposals)
       for more details on telemetry proposals.
@@ -22,7 +19,7 @@ body:
 - type: input
   attributes:
     label: Proposed Config Change
-    description: |
+    description: >
       A CL containing proposed changes to the
       [config.txt](https://go.googlesource.com/telemetry/+/master/internal/chartconfig/config.txt)
       chart configuration.