The programmer's manual is essential for concepts.
Nearly all the clients supplied come with a man page which is installed into /usr/mgr/man/man1 or man6. Other useful man pages are bitblit.3, font.5, and bitmap.5. There is some ambiguity in the docs in distinguishing the internal bitmap format found in your frame-buffer and the external bitmap format found in files, e.g. icons.
The mgr.1 man page covers command line options, commands in the ~/.mgrc startup file, mouse and menu interaction with the server, and hot-key shortcuts available on systems with such hot-keys.
Many of the fonts in /usr/mgr/font/* are described to some extent in /usr/mgr/font/*.txt, e.g. /usr/mgr/font/FONTDIR.txt gives X-style font descriptions for the fonts obtained in .bdf format. Font names end in WxH, where W and H are the decimal width and height in pixels of each character box.