go/analysis: tweak Diagnostic.SuggestedFixes doc

Fixes golang/go#72105

Change-Id: Icaca158250de175a3afcadc218aed8ca617f2ff4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/716562
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

go/analysis/diagnostic.go

@@ -33,8 +33,9 @@ type Diagnostic struct {
 	URL string
 
 	// SuggestedFixes is an optional list of fixes to address the
-	// problem described by the diagnostic. Each one represents
-	// an alternative strategy; at most one may be applied.
+	// problem described by the diagnostic. Each one represents an
+	// alternative strategy, and should have a distinct and
+	// descriptive message; at most one may be applied.
 	//
 	// Fixes for different diagnostics should be treated as
 	// independent changes to the same baseline file state,