Add a "description" field to the label definition

With submit requirements, we'll have some labels associated with submit
requirements while others aren't (for now we name them 'trigger votes').
This change is to provide more clarity/explanation of what trigger votes
mean and show this explicit description in the UI.

Bug: Google b/205810851
Change-Id: Ic7bc1a6f5938958a3c1a6de206ee226b2b1fb320

proto/cache.proto

diff --git a/proto/cache.proto b/proto/cache.proto
index 16e5e95..56aac72 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -448,7 +448,7 @@
 }
 
 // Serialized form of com.google.gerrit.common.data.LabelType.
-// Next ID: 21
+// Next ID: 22
 message LabelTypeProto {
   string name = 1;
   string function = 2; // ENUM as String
@@ -470,6 +470,7 @@
   repeated string ref_patterns = 18;
   bool copy_all_scores_if_list_of_files_did_not_change = 19;
   string copy_condition = 20;
+  string description = 21;
 }
 
 // Serialized form of com.google.gerrit.entities.SubmitRequirement.