Docs: transcoder improvement

I think there's an error in the Python SDK Advanced
Usage page.
Under the Custom Transcoder Objects section:
http://docs.couchbase.com/couchbase-sdk-python-1.2/#custom-transcoder-objects

New to Python and to Couchbase I've been playing around, but was struggling
to get this to work.
I believe the problem is two folds (ignoring the lack of 1import Couchbase`
and `import FMT_BYTES` as this is only a snippet):

• use of `format` instead of `flags` in `decode_value()`
• not converting "long value" to bytes

Please find attached a patch which, if I am correct, I think would save
time, and avoid confusion, to newcomers like me.