doc: document _transform callback takes 2 args

Expands the paragraph in the transform stream
implementation docs about the callback that is passed
to the _transform method to include details about how
two arguments may be passed, error and data.  A code
example is also included.

Reviewed-By: Fedor Indutny <fedor@indutny.com>
Merge remote-tracking branch 'upstream/v0.10' into v0.10