[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
each case.

Reviewers: klimek, probinson

Subscribers: probinson, nwilson, reames, llvm-commits

Differential Revision: http://reviews.llvm.org/D15801

llvm-svn: 257764