docs: Update memory backend docs

[fdegiuli]

Make it clear that the memory backend returns references to values and not copies of the values in the cache.

Fixes #28

[jkrems]

Thanks! Though according to the preview it doesn't render correctly, it shows * characters instead of bold/italic.

[fdegiuli]

Huh, weird. It looked fine for me. Should be fixed now anyway.

[jkrems]

Thanks!