3.x: Add 'Error handling' section to Observable.blockingFirst documentation (#6900) (#6903)

Author: vjgarciag96

src/main/java/io/reactivex/rxjava3/core/Observable.java

@@ -5397,6 +5397,10 @@ public final Single<Boolean> any(@NonNull Predicate<? super T> predicate) {
      * <dl>
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code blockingFirst} does not operate by default on a particular {@link Scheduler}.</dd>
+     *  <dt><b>Error handling:</b></dt>
+     *  <dd>If the source signals an error, the operator wraps a checked {@link Exception}
+     *  into {@link RuntimeException} and throws that. Otherwise, {@code RuntimeException}s and
+     *  {@link Error}s are rethrown as they are.</dd>
      * </dl>
      *
      * @return the first item emitted by the current {@code Observable}
@@ -5425,6 +5429,10 @@ public final T blockingFirst() {
      * <dl>
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code blockingFirst} does not operate by default on a particular {@link Scheduler}.</dd>
+     *  <dt><b>Error handling:</b></dt>
+     *  <dd>If the source signals an error, the operator wraps a checked {@link Exception}
+     *  into {@link RuntimeException} and throws that. Otherwise, {@code RuntimeException}s and
+     *  {@link Error}s are rethrown as they are.</dd>
      * </dl>
      *
      * @param defaultItem