Timber.Tree.log() param documentation is incorrect.

The documentation of the `message` param indicates that it may be null when `t` is not null:
> @param message Formatted log message. May be {@code null}, but then {@code t} will not be.

However, the implementation of `Timber.Tree.prepareLog()` will always append the stack trace to the message when `t` is not null. Hence, `message` will never be null.

Start with the Timber.Tree.log() documentation and inspect Timber.Tree.prepareLog(), which the issue identifies as the relevant implementation. Correct the `message` parameter documentation to match the observed behavior, then verify the resulting API documentation and run the relevant project checks.

Written by the indexing model from the issue text.