Move away from zero-RPS == unlimited-RPS, as it isn't always correct (c…

…adence-workflow#6468)

"Global" RPS limits have a surprising fallback that keeps confusing us:
if the configured RPS is zero, it ignores the limit entirely and only
uses the per-instance RPS (e.g. `FrontendMaxDomainVisibilityRPSPerInstance`).

But since these are generally used in a tiered setup with those same limits,
this doesn't really do anything except repeatedly mislead us that "0" means
"unlimited" because the per-instance-RPS values are generally very high.

So this moves one away and adds some documentation about it.  There may be
others, but they aren't adjusted anywhere near as much as these three.

common/dynamicconfig/constants.go

@@ -598,7 +598,7 @@ const (
 	//
 	// KeyName: frontend.globalDomainrps
 	// Value type: Int
-	// Default value: 0
+	// Default value: UnlimitedRPS (0 triggers a fallback to per-instance-RPS, generally avoid)
 	// Allowed filters: DomainName
 	FrontendGlobalDomainUserRPS
 	// FrontendGlobalDomainWorkerRPS is used to limit "worker" requests (PollFor...Task, RespondTask..., etc)
@@ -613,7 +613,7 @@ const (
 	//
 	// KeyName: frontend.globalDomainWorkerrps
 	// Value type: Int
-	// Default value: UnlimitedRPS
+	// Default value: UnlimitedRPS (0 triggers a fallback to per-instance-RPS, generally avoid)
 	// Allowed filters: DomainName
 	FrontendGlobalDomainWorkerRPS
 	// FrontendGlobalDomainVisibilityRPS is used to limit "visibility" requests (ListWorkflow* and similar)
@@ -628,7 +628,7 @@ const (
 	//
 	// KeyName: frontend.globalDomainVisibilityrps
 	// Value type: Int
-	// Default value: UnlimitedRPS
+	// Default value: UnlimitedRPS (0 triggers a fallback to per-instance-RPS, generally avoid)
 	// Allowed filters: DomainName
 	FrontendGlobalDomainVisibilityRPS
 	// FrontendGlobalDomainAsyncRPS is used to limit "async" requests (StartWorkflowAsync, etc for many "user" APIs)
@@ -3107,7 +3107,7 @@ var IntKeys = map[IntKey]DynamicInt{
 		KeyName:      "frontend.globalDomainrps",
 		Filters:      []Filter{DomainName},
 		Description:  "FrontendGlobalDomainUserRPS is workflow domain rate limit per second for the whole Cadence cluster",
-		DefaultValue: 0,
+		DefaultValue: UnlimitedRPS,
 	},
 	FrontendGlobalDomainWorkerRPS: {
 		KeyName:      "frontend.globalDomainWorkerrps",