[docs] Improve the documentation on committing code reviewed on
Phabricator to trunk.
The previous documentation had a few issues:
- It did not make it explicit that code could be
committed without using the Arcanist tool and how this should be done.
- There was also an implicit assumption on using Subversion
rather than git-svn in the example using Arcanist. The documentation now
explicitly mentions both cases and details how to commit to trunk in