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>

Cherry-picked-from: https://github.com/joyent/node/commit/c8e0bdd7cf628ee2f3f79e5538132467bbc50b4a