docs: improve documentation for merge behavior (#3257)

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/SetOptions.java

@@ -59,7 +59,8 @@ public FieldMask getFieldMask() {
 
   /**
    * Changes the behavior of {@code set()} calls to only replace the values specified in its data
-   * argument. Fields omitted from the {@code set()} call will remain untouched.
+   * argument. Fields omitted from the {@code set()} call will remain untouched. If your input sets
+   * any field to an empty map, all nested fields are overwritten.
    */
   @NonNull
   public static SetOptions merge() {
@@ -68,7 +69,8 @@ public static SetOptions merge() {
 
   /**
    * Changes the behavior of {@code set()} calls to only replace the given fields. Any field that is
-   * not specified in {@code fields} is ignored and remains untouched.
+   * not specified in {@code fields} is ignored and remains untouched. If your input sets any field
+   * to an empty map, all nested fields are overwritten.
    *
    * <p>It is an error to pass a {@code SetOptions} object to a {@code set()} call that is missing a
    * value for any of the fields specified here.