Documentation mentions nested transactions, but unsupported exceptions thrown

http://docs.janusgraph.org/0.1.0-SNAPSHOT/tx.html#multi-thread-tx

When trying to use nested transactions, I get an "unsupported feature" exception. Do we need to remove this doc section, implement that support, or is there a configuration option I am missing to enable that feature (if so, it would be great to have that documented in that section).

Start with the nested-transactions section at the linked tx.html documentation page and trace the reported unsupported-feature exception to determine whether nested transactions are implemented or configurable. Done means the documentation accurately reflects the supported behavior, including any required configuration, or the requested support is implemented and documented.

Written by the indexing model from the issue text.