Correct missing arguments in _EnclodeLOB docs

6b12b8f added $Filename but didn't update the docs.
This commit also added the special return condition without documenting
it.
Document _DecodeLOB.  Improve the _EncodeLOB docs

In particular, call out their return and expected values in case you're
trying to pass the return from _DecodeLOB back into _EncodeLOB, which
RT::Migrate does.
Merge branch '4.0/encode-and-decode-docs' into 4.0-trunk