Add note about references in Out

[MasonProtter]

This is a common footgun with the whole Out thing, so it's good to warn people about it. E.g.

julia> using REPL

julia> REPL.numbered_prompt!()

In [3]: rand(1000, 1000);

In [4]: Base.summarysize(Out)
8000496

In [5]: rand(1000, 1000);

In [6]: Base.summarysize(Out)
16000544

In [7]: rand(1000, 1000);

In [8]: Base.summarysize(Out)
24000592

In [9]: rand(1000, 1000);

In [10]: Base.summarysize(Out)
32000640

In [11]: rand(1000, 1000);

In [12]: Base.summarysize(Out)
40000688

This is an easy way to run out of memory.

Prior art in https://julialang.github.io/IJulia.jl/dev/manual/usage/#Input-and-output-history