toStringz note about keeping references

[Andrej Mitrovic]

On 10/15/12, Jonathan M Davis <jmdavisProg at gmx.com> wrote:
> Anything and everything with no references to it any
> longer should be up for collection.

I think this is fuzzy territory and it's a good opportunity to
properly document GC behavior.

For example, TDPL states that immutable data is always available, and
that a user should treat such data as if it existed throughout the
lifetime of the program.