Where can a PDF of the TPP legend and supplemental materials be found?
Apparently the file you are looking for is FRNTMATTER.PDF, located in the "D" download file.   DDTPPD_201603.zip

(but it still looks like you have to download the 900MB ZIP file just to get that one important file)

The file that @JonathanWalters referenced has very similar information in a much smaller download and a different format.  But there are differences between the two. 

I recommend downloading both the Chart Users guide at the FRNTMATTER.PDF file, and judging for yourself which one (or both) best has the info you are after.