Why are certain commands not documented?

[webmasterpdx]

but these cmds aren't even in the catalog, so the help menu won't work on them.....
They allow you to access a matrix/vector by reference.
e.g.
m:=[1 2 3];
l:=m;
m[1]=<4; // gets translated to array_sto(4,at(m,0));
m returns [4 2 3]
l returns [4 2 3] also
So, I know how it works, kinda, but I'd love to find what else is possible (and undocumented).