mh-command -help
For example, to get a brief description of scan, type:
convolution-259: scan -help
You can also look at the man (manual) page for each MH command. At the end of the man page for mh is a list of all the MH commands:
convolution-259: man mh
The most comprehensive source of MH documentation is the MH & xmh - E-mail for Users & Programmers - (Nutshell). Copies of this manual are available in the Glue Reference Collection in room 0123c of the Engineering Classroom building.