We still provide documents for legacy Market Data APIs in the RFA 7.x package. It locates under the folder.
<RFA 7.6/7.7 install folder>/Legacy/Docs/. It's filename tmsgapi.pdf.
Note that only RDC named user can download the RFA package from
If you still have RFA 7.6.2 package or you have permission to download RFA, you can find the document under the folder.