Document our exciting Matches/NotMatches discoveries on @Placeholder.

PiperOrigin-RevId: 688935702

core/src/main/java/com/google/errorprone/refaster/annotation/Placeholder.java

@@ -79,6 +79,10 @@
  * <em>must</em> contain references to all arguments that <em>are</em> passed to them -- except
  * those corresponding to parameters annotated with {@link MayOptionallyUse}.
  *
+ * <p>Note that {@link Matches} and {@link NotMatches} can be applied to placeholder methods, but,
+ * <strong>and this is a serious health warning</strong>, those predicates will only be matched if
+ * the placeholder method is used as an <em>expression</em>, not a <em>statement</em>.
+ *
  * @author [email protected] (Louis Wasserman)
  */
 @Target(ElementType.METHOD)