[#93] Clarify applicability of special commands for Titan upload

The help page currently lists the special commands available from the new post and comment prompts, but doesn’t specify whether the commands are usable with the Titan endpoint. Checking the source (as of 374faca7bee892540231982b2b3ab5d36419d3a1) the command handling is guarded by a test on 𝚜𝚎𝚜𝚜𝚒𝚘𝚗.𝚒𝚜_𝚐𝚎𝚖𝚒𝚗𝚒. Could the help page could be updated to clarify this?

Thanks.