Improve documentation for ImmutableSetForContains.

PiperOrigin-RevId: 344871950

core/src/main/java/com/google/errorprone/bugpatterns/ImmutableSetForContains.java

@@ -75,8 +75,10 @@
 @BugPattern(
     name = "ImmutableSetForContains",
     summary =
-        "ImmutableSet is a more efficient type for private static final constants if the constant"
-            + " is only used for contains, containsAll or isEmpty checks.",
+        "This private static ImmutableList either does not contain duplicates or is only used for"
+            + " contains, containsAll or isEmpty checks or both. ImmutableSet is a better type for"
+            + " such collection. It is often more efficient and / or captures useful info about"
+            + " absence of duplicates.",
     severity = SUGGESTION)
 public final class ImmutableSetForContains extends BugChecker implements ClassTreeMatcher {