Add a comment to explain why FlagSpec is here; it does rather stand out.

PiperOrigin-RevId: 820624519
diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryQualifier.java b/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryQualifier.java
index 4008b26..21e276c 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryQualifier.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryQualifier.java
@@ -228,6 +228,8 @@
   private static final ImmutableSet<String> INJECTION_FIELDS =
       ImmutableSet.of(
           // keep-sorted start
+          // FlagSpec is obviously not an injecty annotation, but it's commonly used with
+          // FlagBinder, at which point qualifiers on the field are relevant.
           // keep-sorted end
           );