Document that transform and transformAsync propagate exceptions thrown by the function

[cindykrafft]

The Javadoc for Futures.transform, Futures.transformAsync, FluentFuture.transform and FluentFuture.transformAsync says that if the input fails, "the returned {@code Future} fails with the same exception (and the function is not invoked)" (e.g. Futures.java:465-466). As far as I can see, it does not say what happens when the function itself throws. This adds the following sentence right after the existing one in all four methods:

If the function throws an exception, the returned {@code Future} fails with that exception.

This is a Javadoc-only change. catching and catchingAsync (in both Futures and FluentFuture) already say "If, during the invocation of {@code fallback}, an exception is thrown, this exception is used as the result of the output {@code Future}", so they are not changed.

Fixes #2690

Verification:

• In AbstractTransformFuture.run(), a Throwable from doTransform is caught with catch (Throwable t) and passed to setException(t) (AbstractTransformFuture.java:125-130).
• I also ran a small program against the built jar, once with directExecutor() and once with a thread pool. In every case the returned future failed, and ExecutionException.getCause() was the same instance the function threw:
  • transform with a RuntimeException or an Error, whether the input was already done or completed later
  • transformAsync with a RuntimeException, a checked exception or an Error
  • the same checks through FluentFuture
• If an AsyncFunction returns null, the returned future fails with a NullPointerException ("AsyncFunction.apply returned null instead of a Future..."). This PR does not document that case.

Testing:

• ./mvnw -pl guava,guava-testlib,guava-tests test -Dtest.include="**/FuturesTest.java,**/FluentFutureTest.java": 169 tests (160 in FuturesTest, 9 in FluentFutureTest), 0 failures, 0 errors.
• ./mvnw -pl guava javadoc:javadoc: BUILD SUCCESS, and the new sentence appears in the generated Futures.html and FluentFuture.html.