Clearly explain what part of partial instantiation isn't supported.

In (at least) two places we say that partial instantiation doesn't work or
isn't supported.  This has raised confusion so this patch attempts to
explain what exactly is unsupported and why.

doc/reference_manual.texi:
library/list.m:
LIMITATIONS:
    As above.

library/list.m:
    Delete deprecated modes.

    Move partial instantiation states and modes into a private interface
    section.  Note all of the instantiation states are partial, some can be
    safely used for subtyping so I've left those where they are.

LIMITATIONS:
    Include Zoltan's explanation of why this doesn't work for our future
    selves and others.