Hi,
I had the same problem. I found two methods called " compressRTFData/uncompressRTFData " in the docblock.m file (matlabroot\toolbox\simulink\blocks\docblock.m). In case of the RTF_ZIP format you can simply pass the binary/compressed data from userdata.content to the uncompressRTFData function and get the uncompressed data, which can be directly interpreted as RTF.
I hope this helps.
Best wishes