Hi Hans,
There's this...
https://en.wikibooks.org/wiki/Rebol_programming/Dictionary
And you'll see there that WHAT (inexplicably) is for listing all Rebol's 
functions, functions being closest to what you mean by 'commands'. It's 
explained here...
https://en.wikibooks.org/wiki/REBOL_Programming/what
and you'll see it also gives you the source code. (So that's how you 
list all the functions.) Also, this does the same for the question-mark 
(used to show what a function does)...
https://en.wikibooks.org/wiki/REBOL_Programming/%3F
(I don't know which Rebol they refer to, but...
 >> ?? what
and
 >> ?? ?
will give you their source code at the Rebol Console for the version of 
Rebol you're using.)
By somehow combining those two functions, you could come up with a 
script that'd print out all the functions, their description and their 
usage that could be nicely formatted to not waste a lot of paper when 
they're printed out.
Someone's probably already done this, but the trouble with hard-copy is 
it slowly goes out of date. Having one Rebol book is useful though. My 
most thumbed one is the Rebol/Core User Guide.
Hope that helps!
-- Carl.