self-documentation of Control-X j et al

[DLW at MIT-AI (Daniel L. Weinreb)]

The function DOCUMENT-KEY needs more hair to deal correctly with commands
that involve a prefix.  It now does the following:

Control-X j is an alias for J.
J runs the function FOOBAR...

rather than

Control-X j is an alias for Control-X J.
Control-X J runs the function FOOBAR...

This is on the old Xerox system, but it looks from the source like it
has not been fixed; I think this has been in the system since there
ever was self-documentation.