
That's not all it says.  The coverage of modifiers is in Chapter 8
of Volume Four (or chap 7 in older Athena editions), where there is about 
10 pages on how to specify translations to capture
various combinations of keys or buttons.

The Xlib route uses XLookupString and there is a lot of coverage
of how that works in Volume One,  chapter 9.  You should read this
to understand how all the mappings work before trying to do anything
complicated.