detail docs of some important module

[senlong2018]

Hi，I read the rtl and found some important modules do not have a documentation, such as modules in the picture below:

I found the comment such as in axi_demux.v,which means it should have a doc,but I can not found in the repo.

so,I'm apprecatate if you can add the important module's documentation in the doc folder,the important module is:

1. rv_iopmp_data_abstractor_axi,
2. rv_iopmp_matching_logic,
3. axi_demux