Re: [PATCH] add concurrent_abort callback for output plugin

The last sentences there now seems to relate to just the setting of
"concurrent_abort", rather than the whole reason to invoke the
prepare_cb.  And the reference to the "gid" is a bit lost.  Maybe:

    "Thus even in case of a concurrent abort, enough information is
     provided to the output plugin for it to properly deal with the
     <command>ROLLBACK PREPARED</command> once that is decoded."

Alternatively, state that the gid is otherwise missing earlier in the
docs (similar to how the commit message describes it).