oleobj - improve documentation

[decalage2]

Add a clarification about which kind of embedded files are supported by oleobj or not:

• picture files are directly embedded as pictures, not as OLE objects. So it is normal that oleobj does not see them.

• Native MS Office files are embedded into an "ObjectPool" construct, which is not an OLE object either. I have not researched it yet, but it would require a different parser. (the structure of native Office files is "merged" stream by stream, the actual file is not present as-is in the structure). See https://msdn.microsoft.com/en-us/library/dd911041(v=office.12).aspx for MS specifications.

• other files such as PDF (or EXE, VBS, JS, BAT, etc) are embedded as OLE objects, so oleobj works in that case.