Add note about memory of interpolated variables to manual

[mfsch]

As mentioned in #214, I think it would be a good idea to document somewhere that variables interpolated into a benchmark won’t get garbage collected. Feel free to change or move the note.

[vchuravy]

Thanks! That's a good caveat to add.

[mcabbott]

Is this also true of objects created in setup= block? This might be worth also mentioning, as that isn't obviously interpolation. (Especially if it's not true! I'm not quite sure.)

[vchuravy]

Fixed by #265