Improve PODs.
Timeline: POD: write SYNOPSIS.
Timeline: POD: write about callback-style methods. mention future_of().
Main, Timeline: little change the spec of $error in callback.
before: SUCCESS -> $error = undef, FAILURE -> $error is defined.
after:  SUCCESS -> $error = undef, FAILURE -> $error is truthy.
This change is for spec of future_of(), which determines success/failure by $error's boolean value, not by defined-ness.
Due to limitation of Future module, future_of() cannot accept falsy value as failure.
Timeline: POD: add an example to watch_unacked_counts(). Add note that future_of() should never be applied to watch_unacked_counts method.